Here's how I've started tackling this. This strategy works on Normal, Hard and Elite, and doesn't require much gear.
Thanks to people that have run with me and helped refine this, or have helped figured out other things like the aggro patterns of the blades. Major props to the forum poster Varusso that did a lot of research on the blade mechanics in their guide that this took a lot from. Wherever I've put three numbers with slashes between (e.g. 1/1/2), the first refers to Normal runs, then Hard, then Elite.
I recommend you do not attempt Hard until you have managed 5 Normal runs in a row without dying in part 4 (really unavoidable deaths like dying 15 seconds after the last healing-capable character dies don't count). Likewise, I recommend 'mastering' Hard before jumping to Elite. There is a big difficulty increase with each level.
Remember that in this whole fight, noone that dies can be raised. This makes staying alive more important than anything else. If it's a choice between sustaining a stack of Divine Punishment or dodging a blade, dodge the blade even though that means missing out on 3000-4000 damage on the Fiend.
Firstly - Before entering the raid:
If you are a primarily melee class, make sure you have either a weapon that bypasses Harry's DR (this means a weapon with both the 'Good' and 'Silver' flags, which are listed close to the base damage stats on the weapon), or on Normal only, a weapon that does so much damage that not breaking DR isn't the end of the world (like a +4 Vicious Khopesh of Greater Evil Outsider Bane, an Epic Sword of Shadow, or a Shroud-crafted Lightning 2 weapon). If you don't own such a weapon, you can get a basic one - such as a +3 Holy Silver of Lesser Evil Outsider Bane - crafted for you very cheaply by someone with Cannith crafting.
In addition, I highly recommend obtaining a DR-breaking throwing weapon, or a bow and arrows that together break Harry's DR - there are now times that you cannot reach Harry in melee and continuing dealing damage helps. Again, a cheap Cannith-crafted +3 Holy Silver Returning Throwing Axe of Lesser Evil Outsider Bane costs about 100 greater essences to have someone else craft for you and will serve you well in here.
If you are an arcane or divine caster, carry a few mana potions in case things go pear-shaped, but also set a limit as to how many you will drink to try to salvage a run. Also ensure you have at least 1/1/2 three-use 75% damage enhancement clickies for all major spells you will use in the Harry fight.
All players require the following for Harry:
- 30 or better resistance to fire. More is better but very hard to attain.
- If your Reflex save against spells is under 30/32/34 (n/h/e) without it, Greater Heroism. If your Reflex save equals or exceeds those benchmarks, you'll save against Harry's spells on a 2 anyway.
- Something to make them immune to Harry's Mass Hold Monster (Freedom of Movement, WF race, Kundarak Delving Boots). High Will saves are no longer enough as rolling a 1 will stop you moving for two seconds - possibly enough time for blades to kill you.
In addition, the following are recommended for trash:
- Blur (note that Harry sees straight through this, but the trash do not)
- 10 or better resistance to sonic
Less important but still useful:
- Neutralise Poison
- Mass Protection from Elements (highly recommended on low HP characters)
- Silver Weapons and/or Aligned Weapons: Good
- Magic Circle Against Evil
Totally useless spells that you can bypass:
- Spell Resistance
- The other three elemental resists
- Heroes' Feast (the spell effect would be useful for poison immunity at least, but IIRC it's still bugged)
If you are leading the raid, for each buff assign one player (of a class that can cast it) to throw it on the whole raid.
When you zone in, notice that you are in a circular arena. Think of it as a clock face - this will help you quickly communicate locations over voice chat, such as 'Blade chasing me, 9 o'clock outer rim', 'Discoball at 1 o'clock, halfway in', or 'blocking fireballs from 7 o'clock melee range, stay behind me'.
Phase 1: The initial part 4 trash:
People who have run Shroud dozens of times before U12 do not respect this trash and will treat it with the same contempt they treat part 2 trash mobs. DO NOT DO THIS. Every green devil in this part can now kill a careless level 20 character, even on Normal.
There are 7 or 8 mobs visible when you enter part 4 - 3 Orthons, 2 Barbazu (green devils), and a couple of Bezekira (cats). The orthons are harmless but have a lot of HP. The Bezekira can trip you but aren't otherwise all that dangerous. It's the Barbazu that you have to be very careful of.
In all of part 4, each time a Barbazu dies, it leaves behind a Shavarath Blade that will chase a player for a variable time, then disappear. These blades will kill you quickly if you are caught near them. The blades seem to chase the person that had the most aggro on the devil when it was slain, but do occasionally change aggro.
If a blade is chasing you, or if you see a blade and suspect it is chasing you, run away from it and from the rest of the raid. Pick an open space and run in large circles until you are certain that you are not being chased. This might make you hard to target with heals, but you should be taking little or no damage.
This first ~8 mobs only appear once. After they die you will have 4 Barbazu appear at 6 o'clock outer rim.
Never cast Implosion or Wail of the Banshee on these bearded devils. Killing 2 or 3 of them in one place at once will spawn multiple blades - often enough to kill you and every other raid member standing near you. My preferred strategy is to use AoE crowd control spells (Mass Hold Monster, Discoball, Web or anything similar) to get these devils stuck in one place, then have melees run up and kill them one at a time. When one is about to die, call out over voice 'Scatter' so that when the blade spawns, everyone is already running away from it - then everyone that isn't being chased by the blade runs in and takes out the next one. Other strategies used on these devils include using instant-death spells from range (Circle of Death being a star here, especially when a geared Wizard throws it), and leaving the mobs alive and crowd controlled at ~5% and using ranged attacks to deal the finishing blow.
When one devil remains, everyone should activate any spell enhancement clickies, then kill the last mob. It is very important here to establish exactly who (if anyone) is still being chased by blades. The biggest single cause of raid wipes in part 4 is someone 'leading' a blade into the main melee pack, or to the healers.
Phase 2: Harry on the ground:
A quick note: This strategy is fairly conservative, in that it prioritizes survival over DPS and takes few risks. You can wait longer before pulling out of the blades, and will need to if you intend to set any speed records. In addition, it asks you to keep track of time - one way to very easily do this is to have a melee use their action boosts at the start of the fight and again immediately when they expire - if you do this, the second one will expire after 50 seconds.
Once the last bearded devil is slain, Phase 2 begins. Refer to the following diagram: . The big red blob is Arraetrikos, who will drop in a few seconds into the phase.
During those seconds, melees should split themselves between the two spots marked M1 and M2 on the map, with at least one person at M1, and players that want to avoid Harry's melee attacks (many arcanes and divines, plus some rangers) should run to H1. H2 isn't safe at the start of this phase, so noone should go there yet. Note that everyone is on about the 1 o'clock to 7 o'clock line - this is deliberate, it lets those at H1 use those at M1 as human/halfling shields from Arraetrikos' fireballs. Lower level clerics in particular can be knocked alarmingly low in HP by one Meteor Swarm, so the shield will help a lot.
Remember, if you are still being chased by blades, DO NOT go into position yet. Instead run to a position that noone else is, perhaps 10 o'clock halfway out, and run around in circles until the blade chasing you despawns. Of three Shroud wipes I've been in since U12, two have happened because someone led a blade to the main melee pack, and the combined damage of Harry's melee attacks and spells plus a blade was just unhealable.
While in the initial position, melees hit Harry with everything you have. Arcanes should throw their highest damage-per-mana spells on the fiend - Eladar's Electric Surge, Niac's Biting Cold, as well as a Cloudkill to provide a concealment bonus to melees. Keep these spells up throughout all of the phase, and if you think Harry might be one-rounded, throw more spells into the mix such as Polar Ray, Acid Rain, Black Dragon Bolt and the like.
One divine should target one of the higher HP melees and throw Quickened Mass Heal on them basically as soon as the spell is off cooldown (Harry does almost exactly the right amount of damage for this to work perfectly), and the other divines should throw single-target Heal and/or Mass Cures on players that are at imminent risk of death, such as lower-HP melees that fail a DBF save. When you have some practice with the basics, add in casting Maximized, Empowered Divine Punishment on Harry, but don't worry if you aren't confident to do this in your first runs.
For about 20 seconds, everyone will remain in these positions. When Harry drops in, about a dozen blades spawn on the far outer rim of the arena, and from there they will spiral in towards the middle. (This is why you can't stand at H2 yet). After 20 seconds or so, H2 will have been passed by the blades, and they will be getting a little closer to H1. At this point, those at H1 should run in a straight line to H2. This involves crossing the line of blades - on Normal, you can ignore them and accept that you'll take about 200 damage crossing them. On Elite, you have to time your dash to avoid the blades, or else you might take 600-800 damage instead.
At around the 40 second mark (this is halfway through your second action boost on a melee), someone should call out 'Collapse' in voice chat. At this point, those melees at M2 should all move to M1. Healing will become a little more intense at this point as Harry will be hitting every melee party member with every one of his swings, rather than him facing away from you half the time that you are toe-to-toe with him.
If total combined raidwide DPS (damage per second) is more than 2400/3600/6400, Harry will drop within 50 seconds and the rest of this guide can be ignored. This is only the case in a minority of groups, so expect to have to run out of the blades.
At around the 50 second mark (just after your second action boost expires), the blades will get uncomfortably close to M1. At this point, the raid leader calls 'Last heal in 3.. 2.. 1..'. This is a signal to the melees - the next time you are at full health, run directly (in a straight line, possibly crossing the blades) all the way to H2. As you will likely take some damage on the way out, you should wait until you get the final burst of healing before running out.
Stand as a group at H2. Harry's Fireballs will hit the whole raid, but they are easily healed through with just the Mass Heal spell. Everyone should do whatever they can to damage Harry from range - Holy Silver throwers, Holy bows with silver arrows, or spellcasting. Rangers (including Tempests) should use Manyshot here. You'll be under light fireball fire, and if you stand still you'll take one more hit from a blade as it moves away from Harry and back to the outer rim, but it's all easily healed. After that, it's on to Phase 3.
Phase 3: Harry overhead
Harry will fly overhead strafing the raid with fireballs. Stand at H2, and noone will be hurt. Use this time to heal up without using SP (Radiant Servant aura, and/or Heal scrolls) or to take a quick breather. It's over quickly though - phase 4 starts in a few seconds.
Phase 4: A simpler Phase 1
After Harry's final strafing run, you'll be back to the second part of phase 1, with four barbazu teleporting in. Dispatch them carefully as you did in phase 1.
Phase 5: Harry + Gnolls
This is a repeat of phase 2, except that at the start, eight Gnoll Idolaters will appear, two from each of the portals on the outer rim (the blue boxes in the arena diagram ). These gnolls have just a few hundred hitpoints, low saves, and no attacks - instead, while they are active they heal Arraetrikos. They should be killed quickly with Wail of the Banshee, Implosion, Circle of Death, Finger of Death, or even melee attacks, but while you kill them, watch the blades.
Usually you will want to assign gnolls to specific players. 17th level or higher Wizards can be comfortably assigned four gnolls (Wail on two, Circle of Death on two, Finger to kill one that saved, Power Word Kill as a backup for the rare times that one survives all of that), and other DC-based casters should be assigned two each to kill. If you are assigned gnolls, kill them and ask for help if you have trouble, then once you've dealt with yours, help others with their gnolls, then get into position and turn on Harry.
Warning: The gnolls will spawn in areas that are hit by blades early on. Keep an eye out for blades while killing the gnolls. If you wait ~2 seconds after the gnolls spawn, they will move a little closer to Harry and out of the immediate mincer.
Healing can be challenging while gnolls are up, as players will be spread out more than usual and non-melees will be copping occasional Fireballs that would usually be blocked by the main Phase 2 strategy. That's the reason you've been conservative with mana so far, using mostly Mass Heal on the melee pack during Phase 2 - hopefully you have some mana to use for the first 10 seconds of this phase.
Once the gnolls are down, revert to the usual part 2 strategy. Phases 3, 4 and 5 repeat. Arraetrikos has 120k/180k/320k HP in part 4, so he shouldn't take too long to kill unless you have a disaster (such as eight people dying in a lagspike, or someone casting Wail of the Banshee in phase 4 and causing 3 blades to spawn on top of the raid). If that happens, you'll probably wipe.
A word on part 5:
Part 5 is much easier than Part 4, despite Harry having significantly higher stats in part 5. However, about three minutes into the Arraetrikos fight in part 5, blades will start causing you serious trouble. Treat part 5 as a race - either you get Harry down inside ~3 minutes, or all hell breaks loose. So throw everything you have at him and don't hold back.
Advanced approaches to speed up the fight:
Not recommended until you have the basics downpat, but you can speed Phase 1 and 4 up with *extremely* careful use of Implosion or Wail of the Banshee. You can put up a discoball or web, trap multiple barbazu in it, have everyone else leave, then throw one of the AoE death spells as you are running away from them. They don't spawn a blade for a split second after you kill them, which can buy you time to get away if you have quick reflexes and know exactly what you are doing.
After the group evacuates from blades in phases 2 and 5, SP can be conserved if one person with Evasion (or just a high Reflex save and decent HP) blocks fireballs by standing ~30ft closer to Harry than the main group.
Favored Souls with the Angel of Vengeance 1 PrE bestow a nasty stacking debuff on mobs and bosses that hit them in melee, lowering their Fortification and making them more susceptible to Light damage and Holy weapons. Many FvS players don't realise that they can give this effect to another player instead of themself - so if you don't feel you have the hitpoints to stand toe-to-claw with Arraetrikos, use your Name Champion ability on a sturdy melee, and then transfer your Crown of Condemnation to them. Arraetrikos attacks fast enough that he will frequently suffer multiple stacks of this debuff, occasionally even a full five. If your FvS doesn't have AoV1, respec them now and pick it up, as killing a boss 5% faster is at least as good as stretching your SP 5% further.
If you find yourself the last person standing in a disasterous run, you can dodge Harry's fireballs indefinitely by continually running circles around him at any range. He simply cannot hurt you if you stay out of melee range, don't stop moving and keep forcing him to turn to look at you. (Of course the blades can, and so you'll need to change the radius of the circles you run around him). If he's very low on HP - say under 5% - you might be able to finish the fight off solo with Eladar's Electric Surge, Niac's Biting Cold, Divine Punishment, or other highly damaging ranged abilities such as Manyshot that let you do damage while remaining fully mobile. If you succeed in killing him, you will have a very impressive screenshot to show off, although raid members won't have an opportunity to shrine before part 5 and will need to succeed in part 5 to get their part 4 chests.
It really speeds up this part if clerics and favored souls use Divine Punishment on Harry between heals. After the second cast, each additional time you cast the spell (max-empped) it will do about 1.5% of Harry's total HP in damage on Normal (7 ticks of ~250 damage each, and it does much more damage if you spec heavily for it). The spell takes practice to use while healing, but is well worth familiarizing yourself with - you'll be doing this in other raids and especially in 6-person Epic boss fights.
A final note
Don't be afraid to wipe. The most fun Shroud I've ever done was one of my first attempts after U12 hit. It was on Hard, and my Wizard was the last one standing with Harry at 40% or so. We didn't kill him that shot (I wasn't willing to burn through 15-25 mana potions to kill him and deal with gnolls 2 or 3 times, and wasn't sure I'd do it flawlessly - any mistake would get me killed), but the failed attempt was a lot of fun.
Having fun is more important than getting a large ingredient.