I - Survivability
1 - Invisibility/sneak - sneak costs too little ap at high level, enemies don't know how to counter, mage invisibility spammable each turn, sneak spammable each turn.
I don't think the ap cost is that much of a problem, sneak is broken when combiner with other stuff.
I think enemies should just have something in their rule-base to do when they see no enemies during combat (Tip: use AoE/damage/effects(Midnight Oil/Slow/etc.), retreat and get cover (get away from the last point they saw the opposition, forcing players to walk a lot to get to them), use far sight).
Spammability is directly related to glass cannon. If you use invisibility every turn, you are probably not being that useful UNLESS you have glass cannon. Sneaking probably should have its distance rescaled.
AP cost is more related to Guerilla, and yes, enemies should have counters to invis, both AI and rules wise. Sneak definitely could use distance rescaling, that's a good idea. If will become more suitable for archers in combat in this case. Sneaking at 1m distance from the enemy looks a bit stupid.
2 - Elemental immunity - too easy to get, while shouldn't be a thing at all
Right now super hard-cap at 80 is pretty ok (environmental effects like rain reduce cap to 60). Also, this is only really a problem if you get over 150. 110-130.
Elemental Absorption is a bit crappy because it's mandatory, way better to use Elemental(Air, Water, Earth or Fire) Shield
If the enemies dispelled the shields and debuffed the resistances it would be good, however right now it takes 1 spell to make you invincible to a large portion of the damage in the game. Still much better than it was.
4 - Blocking
I don't know anyone who achieved blocking higher than 80%. I find 80% as cap pretty fair. Maybe 75 or 70 could be ok. I think it's one of the good differences between 2H and 1H. 2H users get more damage. 1H gets better defense. However, if the ratio is equal (the amount of extra damage 2H deals is equal to the amount of attacks a character can block), then people will probably ignore shields. I'd go as far as saying that blocking less than 50% is not that big of a deal at end game and people would be better off dealing more damage.
I'm pretty sure you can get 90%. Not sure about 100%, but I wouldn't rule that out either. 2H deals roughly 20% more damage per ap. Now 2h is much more effective with fixed price skills like WW and flurry, which imo is a design oversight (such skills should be adjusted to the ap cost of the weapons). 75% cap sounds fair, because 90% blocking is a little bit too much.
II - Damage
Main offenders:
-Guerilla
-Bully
-Rage
-Oath of desecration
-Rage
-Stacking +weapon skill items
-Resistance debuffs
Guerrilla is ok. However i think that sneaking in heavy armor should be impossible. Also, if you mess with Sneaking you better help rogues and ranger with their problems (backstab positioning and animation arrow dodging). Maybe make the minimum AP cost = 2.
Bully is a good talent, maybe reducing the bonus a bit. 30% or 25% seems ok.
(I worry about archers in Cyseal if these first two are changed too much)
Rage - it's ok the way it is. I don't think there is a good way to slightly reduce its power. People mostly use it before flurry and just after enemy got stunned or knocked down. also, it's another power that if nerfed too much will become useless.
Oath of Desecration - It's ok. Move this to Skill level 18. Or at least 14. Put Summon Undead in its place.
Resistances shouldn't get negative. So there should be 3 states for Enemy Resistances:
1 - Normal
2 - Lowered (-20)
3 - Nullified (-total)
And none of them can be simultaneously affecting anyone. So if you use Lowered before Nullified's effect is over you just wasted AP.
Good idea with the resistances. I still think rage and oath should not stack, the result is a little bit too much. 2 AP for stealth sounds fair with the other nerfs.
III - CC
I see no problem with CC. However, I think enemies should use CC more. Maybe try to make them do magical combos too (Rain + Frost/Lightning or Poison + Fire, or Teleporting me to a mine that they can see, or using more summons to raise their numbers).
Finally, getting some stupid behavior out of the AI system could be a priority (no more fire elementals hitting each other for nothing).
Also, a question, how the hell they were able to capture that Ice Elemental in Hiberheim if their attacks are Ice based? Did Boreas himself use his newly acquired staff to capture a crappy ice elemental?
The problem with cc is that you can have such high chances of it to succeed, that without diminishing returns you can chain cc everything until it dies. To me this looks like an "I win" button. Enemies OTOH indeed can use smarter AI and cc related tactics, if they did the game will become much more tactical and challenging, especially if you remove permaimmunity items and talents.
