The computer doesn't have Earth Magic, meaning antimagic won't protect it from Berserkers (!)
When only Nags are left against Archers/Hydras and Bulls, you can start a cycle: Blind – raise towers – Hydra attack/strike – on the Bulls' turn, Blind.
It's important that by the beginning of the cycle, the attacking units are ALREADY faster than the Nags.
Because during the cycle, you won't use anything else except Blind.