Well nowdays this is my fav quest, although many ppl hate it, with a bit of practice it can be easy if u have the right spells/habilities, just need to know what to do and when to do it .
Im gonna try to describe how to beat this way with a minimal of spoils, so the objective of this is just guide around to make it easier and you dont get lost on the quest.
Remember you need to broke the portals to travel beween inferno and our world, or in some cases you must need to kill a monster called acolyte of the flame wich when died transports you from our world to inferno. In any case you must surround the target by all your party members before beat it or your party will be divided and probably slaughtered by the mobs.
The objective is kill the guardians of the flame (fire elementals & undead fire elemental) to wake up cinnie (or whatever name is last boss) and confront him to gain his confidence to accept help you beat his master the black abbot.
Quest walkthrough (trying be spoiler free)
So im gonna name the portals by letters (A,B,C, etc) so u will understand better where to go and too im added i to reference if its on inferno, so portal (Ai) will be the first portal on inferno side.
Im gonna skip how to reach the quest, so im assume you are just inside and didnt moved.
You will see 3 paths, south, west and north. Choose west path because you may do all the quest straight forward from this path. Go to the first portal (A), beat it, welcome to inferno, take the right path stairs upwards (south path), beat the portal (Bi), light the torches and defeat the south guardian, go back to the portal(B) and beat it, walk back and reach the 1st portal(Ai) dont beat this one, so now move to the left
path (east), jump downside the cliff, and reach the portal (Ci), beat it, go forward and lit the torchs, defeat the west guardian, go back to the portal(C) beat it, just ahead of it are the torches, unlit them and defeat the undead guardian, back to the portal (Ci), beat it, and now head to the left path, there you will see portal (D), A shrine is located at left path, but read the shrines references to know how to get there, so im asume that u dont need shrine so just break this portal, else if u need shrine after used it back to this portal on inferno side, from portal (Di) inferno side, now u need go the right path, so then u will see path split and stairs, dont take the stairs yet, not needed really, go to left and jump down. There will be unlit torches, so use ur detect doors and u will find portal (Ei), beat it, open the door, lit torches, defeat north guardian, use portal (E) to back into inferno, unlit torches, defeat undead north guardian, climb the boxes and reach the path at north side, u will be in portal (Fi) and u will notice a key through a locked door, if u just want beat the quest ignore this and go through the stairs, else read about optional chest info on chest section. So there you will reach portal (Gi) beat it, and go ahead, there will be a path split but dont worry you see closer another objective, wich tells you were to go, so lit those torches and defeat the east guardian, now move stairs up, you will find portal (H), and closer is another shrine, again read shrines reference (if u used it just now go east side from shrine inferno, and jump down the chasm), so use the portal (H) to go into inferno, go right path and you will find a chasm, jump it unlit torches and defeat east guardian, follow the path until reach the center room, you will see some big torches on each corner, the one lit will tell you where is the way to the left undead guardian, remember is south, so go there (you will see the lit torches from the pathway anyway), unlit them and defeat him, now that you have completed the 4 guardians, prepare yourself and head to the center to defeat cianni, Good luck when he died the chest appears on center room (inferno side). If you still want the optional chest use DD to head back the begining, read the chest section.
--End of walktrough----
POSIBLE SPOILER, ALERT!! from this point, dont keep reading if u dont want be spoiled.
Equipment recomended (although this is handy can be done without have anything of this)
-1 weapon to fast beat the portals (unanimated construct) so im recomend greater construct bane or an elemental great axe, after all you only gonna miss hit it with 1, or still any weapon u dont like much will works fine because breaking the portals will get damaged a lot.
-1 Disruptor, as any mob that died on our plane goes to inferno as undead, so it will be good weapon
or instead you will like some greater undead bane or holy burst weapon , and too the undead fire elementals can be disrupted although
the chances to sucess are low, but 1's happens to them
-1 Banisher, on our plane the elementals can be banished although is hard to do it.
-1 Paralizer, on our plane almost everything can be paralized and on inferno the mephits can be paralized, its usefull
-1 Wounding weapon, that will help you a lot to kill fast some anonying mobs
-1 Wand of fireball to lit our plane flames to wake a guardian.
-1 wand of cone of cold to unlit inferno flames to wake a undead guardian.
Fire resistance/protection - bosses
Cold resistance/protection - bosses
Electric resistance/protection -casters
Death ward -casters
Freedom of Movement -casters
Fireball/firewall - mobs/torchs
Cone of Cold - mobs/torchs
Icestorm - does good dmg to bosses
heal/mass heal - guess that it dont need explanation.
Detect secret doors or good spot.
Dimension door optional if u wish go for optional chest.
Quest specific things.....
On our plane you need lit the torches, the best is use fireball and target to the center of the 4 torches, where is a strange draw in there, so you will lit the 4 with just 1 shoot, but it needs lot of practice, so its ok if u do 1 fireball per torch, or use firewall if u dont have fireball but keep in mind that fire may heal the guardian.
On inferno plane you need to unlit the torches, use cone of cold, so if u stand closer of a torch you can at most unlit 2 at time, and too need practice.
This quest can be done still if you dont have a caster or wands although its harder. You must farm mephits from the closer portal and let they follow you to the torches and let the mephits breath over each torch.
There are 2 resurrection shrines and 2 rest shrines, although if that is not enough and u dont care about xp penalty for reenter is not so far the orchard forest shrine, not many mobs in way if u stick at the path.
Our plane have resurrection shrines guarded by some mobs and with them an acolyte of the flame
At inferno plane have the rest shrines exactly on same place than resurrection shrines but the path is closed by a door wich can be open only by inside, so to open that shrine you need kill the acolyte of the flame on normal plane (and must be done on the area surrounding the shrine) and when it died it will teleport you into inferno, so you must defat/disable him and open the shrine for everyone else.
*Chests (spoiler alert)
There are 3 chests, 2 of them are located on the area of the rest shrines, so you will have no trouble with them, and last chest is the optional one where the shield piece may appear, this chest is locked and the key is located on another place of the dungeon, so you must get the key first, then head to the chest. So both the key and bonus chest are located close to the entrace but at inferno plane well protected by closed gate area, you must access to them like you did to the shrines through killing an acolyte of the flame, but they have an extra protection, since the area where they are is too closed by door on our plane. You need wait the mobs open the door for you and survive their ambush . Key is located by teleport on north path, you must wait 2 min there or break the portal twice to get the ambush, kill the acolyte on the enclosed area and be teleported to inferno, kill acolyte again and pick the key. Bonus chest is located by west path, left branch of the path, wait at door 2 mins, and door will open, kill the acolyte on the enclosed area to teleport into inferno, kill him again and unlock your chest and pray to loot gods for the shield piece.