Missed the first ten days of this thread, sorry if this has been covered.
Prior to MOTU, it appears that the quest designers could tune multiple variables in statting mobs. Consider a monster like a Kobold Shaman in Waterworks.
On Normal, they have a few fairly nonlethal spells, and a (quite dangerous) low DC Niac's Cold Ray.
On Hard, they have the much more dangerous Hold Person
And on Elite, they have (assuming no guild buffs) the terrifying Lightning Bolt.
Their melee combat prowess sucks on all difficulties, but they do significantly more damage on Elite and have much better To-Hit.
In addition, mobs had very varied durability within quests. Consider Rainbow in the Dark - that quest has very high HP Gelatinous Cubes, low HP fragile (but deadly) Barbazu, durable but not very dangerous Orthons and Stone Golems, and a few other mobs.
Contrast this to a post-MotU monster such as the Drow Planescallers in Reclaiming the Rift or a Dire Bear in Thorn and Paw.
On each difficulty, these monsters behave the same. The only difference between an EE Planescaller and an EN one is their stats.
In fact, I've been able to ascertain - through the monster manual - a good deal of how the post-MotU system works.
First, you design a generic 'Epic' version of a mob (or for quests that were Epic at the 20 cap, you use the one that was designed then).
Next, you modify the CR - 60% of base for EN, 85% for EH, 125% for EE.
Finally, you derive (using formulas I can guess at but do not know) all of the monster's stats - HP, spellpower, attack damage, saves, ability scores, etc - from its CR and the 'Old Epic' stats.
In particular, I can tell from experience that (before dungeon scaling) EN red/purple nameds have 25% of 'old epic' HP, EH have 65% IIRC (need to check that) and EE 135%.
The problems here are:
- Mob spells do not vary with difficulty any more. As a result we see ridiculous things like Epic Casual Chamber of Raiyum mummies using the devastating Greater Dispelling --> Destruction combo (which is fine in EE or EH but not suited at all for solo players i.e. the whole audience of the EC difficulty)
- The mob stats are not designed according to the capabilities of the characters that are the target audience of each difficulty.
- There are a few player abilities that are blatantly overpowered at the moment, and monster stats from the Druid's Curse onward have been balanced around the assumption that EE parties have these cookie-cutter, legitimately broken builds. This then utterly invalidates character builds that are not designed around Furyshot, Master's Blitz, Shiradi, or a couple of other less powerful but still borderline viable builds. The decision not to nerf those design mistakes has led to a massive indirect nerf to all other builds as monster stats have risen to challenge the broken builds.
- It is not the case, and has never been the case, that EE-oriented characters have spell DCs 15-20 points above those of the audience for Epic Hard, yet monster saves are regularly 15-20 points higher. Ditto for SR figures.
- Monster attack damage has scaled so high (on EE) that probabalistic damage mitigations (AC and Dodge) are unreliable and so PRR and HP become everything. However, players have already hit the point of sharp diminishing returns on PRR.
What you need to do is generate monster stats and spell lists completely separately for each difficulty.
Get top 10% players to run a quest against your initial attempts at EE stats, and listen to their feedback on how difficult (or easy) it is. Adjust monster damage for telegraphed special attacks through the roof, increase monster movement and attack speed, and tune monster saves so that a max-geared character with all +DC past lives except Completionist and all available EH or lower gear can hit monster 'good saves' 25% of the time and 'bad saves' 75% of the time (factoring in SR as well, if appropriate).
In addition EE monsters need tools to mitigate player kiting that are not needed on lower difficulties. Crippling arrows, the Horrid Wilting spell for casters to use against point-blank circle kiting players, large AoE cleave attacks that trip (with a save) on a vorpal, Dispel/Slow effects to slow players down, and various other tricks like that. Or just have the monsters spawn with Haste.
Finally, really amplify monster strengths but don't plug their weaknesses. An EE rogue trash mob should not have 6000hp. But it should hit like a truck - there's nothing wrong with them having 75% fortification bypass, 150 base damage, 150 sneak attack damage, and a 15-20/x2 crit profile, alongside a mere thousand hitpoints.
Get 11th-50th percentile players to run a quest against your initial EH statted monsters, and adjust to their feedback. On EH, telegraphed special attacks should be dangerous but not deadly (think on par with Heroic Hard Searing Lights in Running with the Devils - those are attacks players should be strafing to avoid but can easily survive a couple of hits from).
Get 51st-90th percentile players to give feedback on EN, and keep it forgiving.
Then get bottom 10% players to give feedback for EC. (This really should be renamed "Storyline Mode" because of the demeaning nature of the term 'casual' in online gaming, but that's a different matter).