What you just described is a flaw in the AI not really the system itself.
There's nothing wrong with the AI. It would be about as effective against a human opponent.
Assuming that I don't stun-lock the opponent on the first turn, the opponent has 3 options:
1. Advance towards me, take damage, lose magic armour, enter stun-lock.
2. Stay still and use any attacks he can do without moving (meanwhile I can focus all my attacks on any characters that still have magic armour until I can force him into a stun-lock).
3. Reply with the same strategy, and turn the entire battlefield into a wet mess, and we constantly electrify the water until one of us successfully stun-locks the other.
Of course, some characters have moves that allow them to charge/teleport, and these can complicate the strategy a little. But once they're stun-locked, it's over.