[K]

When someone is casting Master Enslave, they use either a Pretender or a communion.

If they use a Pretender, your best best is to kill that unit before it casts. The AI likes targeting high HP and large units, so it's not hard for a defender to kill that guy dead before he can cast on round one if you use the good attacking spells.

If they are using a communion, killing off the communion members with area spells works.

Otherwise, sending thugs and SCs with high MR is a fine way to kill a Master Enslave Army. The spell is MR+, so some Banelords with a few MR boosting items basically means they can't be affected.