Hi Michael,
I ran ACKS to max level twice and haven't had this issue. I believe you when you say that you do. So I want to figure out where we are differing.
Here are some sampel monsters from a high-level dungeon in my home campaign:
Sothgar the Death Knight: Mummy Fighter 14; Move 240'; AC 17; HD 14; HP 75; #AT 2 (permanently Hasted) two-handed sword -6+, Dmg 1d10+11, Save F14 (+2 on saves); S 18, I 13, W 13, D 16, C - Ch 14; Proficiencies: Combat Reflexes, Commander, Weapon Focus (swords), Fighting Style (two-handed fighting); Permanent Enchantments: Detect Invisible, Haste, Protection from Normal Missiles, Protection from Good, Resist Fire; Equipment: Plate Armor +3, Two-Handed Vorpal Sword, Cloak of Displacement
(He's a classed NPC who was transformed into a Mummy and retained his class powers.)
12-Head Regenerating Spydra: Move 120', AC 5, HD 12, hp 96, #AT 12, Dmg 1d10 + save v. Poison or Die, Save F12, loses 1 head per 8hp of damage, grows 2 new heads 1d4 rounds after losing a head
(this is a crossbreed between a poisonous spider and a regenerating hydra)
Gorgan, Dire Orc Chieftain: Move 150', AC10, HD10+3, hp 67, #AT 1* warhammer -2+, Dmg 2d6+4, Save F10; carries war hammer +2, wears plate armor, ring of fire resistance, potion of speed, and potion of giant strength. He will imbibe the latter when confronting opponents, giving him a movement of 300', two attacks per round, and each hit does 4d6+4 damage.
Dire Trolls: Move 120', AC 8, HD9+9**, hp54, #AT 3 (claw/claw/bite) 0+, Dmg 1d10/1d10/3d6, Save F11, Regenerate 3hp/round
A few explanatory notes:
- I absolutely assign magic items to intelligent high-level monsters: If magic items are in their treasure hoard, they are used!
- I generally assign proficiencies where relevant to classed NPCs, at least where I think it will make a difference.
- Weapon-armed intelligent monsters are allowed to take advantage of game rules; if they fight with two weapons, their attack throws go up by 1, if they fight with spears, they can charge for double damage.
- For high level play, I made a whole set of "dire beastmen" that are berserkers (+2 to hit and immune to fear) with bonus HD and AC to use as "grunts", and created hideous crossbred monstrosities like the Spydra.
- High-level casters can and will buff up their pets and minions. Inspire Courage and Bless offer +2 to hit and damage; that makes a big difference, especially at the margins (e.g. hitting on 17-20 versus hitting on a 20).
In actual play, the highest AC I've ever seen on one of my PCs was an AC of 19. Sothgar needed a 13+ to hit the PC. Most of the PCs ended the campaign in the 12-15 range, which would have been a 6 to 9+ for Sothgar, 10 to 13+ for Gorgan, and 12+ to 15+ for the trolls or spydra. All of which hit very hard and had multiple attacks.
Marcus, a 14th level fighter, had (as of session 93): plate armor +4, cloak of displacement, girdle of giant strength, gauntlets of ogre power, ring of regeneration, ring of water walking, +3 vorpal two-handed sword, and boots of speed. His AC (with DEX 15) was (6+4+2+1) 13. He hit very, very hard of course; 2 attacks per round with his two-handed sword at -5+ / Dmg 2d10+8.
Farlaghn, a 10th level spellsword, had: plate armor +3, ring of protection +3, ring of fire resistance, sword +3 defender, sword +2 defender, wand of lightning bolts, wand of polymorph. His AC (with DEX 16 and using his swords for defense) was (6+3+3+3+2+2) 19.