Lots of times the AI will go with something sub-optimal. Without going in to the whole making the ai better debate the best thing to do is to script the win before the AI takes over
Now, just from the description of the fight, Banish doesnt sound like a horrible spell. 16 slaves means your masters had +4 to holy level. Spamming level 6 and 7 banish doesnt sound like too terrible a spell to cast. Thats +2 penetration with 0 fatigue and a huge area. Paralyze would also seem to be a good spell to pick if targetting the Tarts.
Saying that, I think unfrozen are the best summons in the game and are often underestimated especially in a cold dominion. Good hits, superb MR, good damage(some even come with magic weapons) If your opponent had cast just an antimagic that boosts their MR to 20 and they are gonna walk thru even serious banishment spam.
Of course the better spell the ai could have switched to instead of banish is witherbones.
Depending on what type of troops you have as blockers you can rarely go wrong with stellar cascade spammed with 32+ mages.