xw, you gotta post that build. Freebie 3d6 + 5d6 sneak dealing arcane blaster. Sounds fun. Just guessing was UR 12 / Wiz 6 / AT 22 based on BAB break point & AT L22 epic bonus feat.
Would suggest Beguiler (Disable Twerp, Pick Nose as class skills), but spell list is absolute crap. No IGMS, Avasculate, Avasc Mass!
One thing that looks strange (WotC geniuses) is AT doesn't do much past L10. No more +1d6 Sneak boost every other level & epic feat progression @ 1 per 4 is worse than most. This essentially removes incentive to go higher, except it's a PrC and gets to epic @ L11 vs L21 for base class.
Since AT is half BAB anyway, progression is the same as any base Arcane caster that gets L9 spells. 3/4 BAB would have been nice, WotC.
Eldritch Knight = full BAB-full caster Arcane; Alaghar, Hospitaler, Master o' Shrouds, Fist o' Raziel, Sacred Fist are all full BAB-full caster Divine. Hos' gave tons of bonus feats, Sacred Fist has uber-class boosts, Master o' Shrouds has semi-infinite uber summons.
WotC