step 1 - get AoE damage units (mages w/ maelstrom, ritualists with RoF, chalid, etc.)
step 2 - get stack defenders - high strength units with lots of first strikes/defensive strikes - crossbowmen w/ blitz work very well here, as do illusionary units (gibbon and earth elementals, etc.)
step 3 - get your pwnage units - units with high attack, movement, blitz, etc.
step 4 - get healers - several units with medic 3, so they can cast heal once you're done attacking
step 5 - get blinding light/slow, and cast it on all enemy AI units
Now take and combine these guys, go forth, smash and burn all his cities to the ground. Don't bother worrying about capturing them - he's the last guy left, right? If you encounter a large AI stack, then fort up on a hill or something, cast AoE on them, blinding light/slow, and pick away at them. Once you've killed the primary stack, you can just mow down the rest of his cities with little trouble.
Alternatively, spam champions, or whatever you want to use until your stack is bigger than his... good luck with that one
-Colin