Abazigal, not Amelyssan.
Stats:
Abazigal: Level 30 Kensai/Mage, HP 354 THAC0 -33 APR 5 AC -18
Yaga-Shura: Level 25 Berserker, HP 406 THAC0 -18/15 (two weapon fighting), APR 5 AC -9, also has large to huge resistances to almost everything that decay over time.
Illasera: Level 18 Ranger, HP 266 THAC0 -26 APR 4.5 AC -9. In my game she has upto level 6 spells but that might be because I have expanded spell progression mods and I think SCS reorganizes spells for enemies taking it into account.
Balthazar: Level 30 Monk, HP 338 THAC0 -17 APR 8 (perm improved haste) AC -14
Sendai: Level 26 Cleric/Mage (Cleric/Conjurer in my game, that might be due to SCS randomly giving mage specializations though), HP 217 THAC0 4 APR 2 AC -4
Melissan: Level 30 Cleric/Mage, 1018 HP THAC0 -15 APR 5 AC -14, also has high resistances to most things. Stats go down the more of the others you've killed.
The absurdly good THAC0 (considering your base AC caps at -20) on Abazigal and to a lesser extent Illasera and immunity to timestop is all that really bugs me. Inflated HP and the resistances can get annoying, but for the most part if you are safe defensively your fighter will be able to output like 200 damage per round. Even if that's cut in half an unprotected Melissan goes down fast when 3 people are wailing on her.