I was playtesting your game and used repel to destory a bunch of rank 1 goblins. I have 2 levels in caster. It seems broken. It might be better served to allow the destruction of enemies at higher levels, maybe 4+
A level 2 spellcaster would not be able to destroy rank-1 creatures. The rank must be less than half. Even at level 6, you may only destroy up to rank-2 creatures.
It's also worth noting the caster may choose to destroy, he is not required to. Use your characters moral code as a guideline.