Actually, Soul Eater can reduced by 8% instead 20%, if you use Darkside to boost the strength of the attack.

Clemency has it's own penalty of (A) a ridiculous MP cost for a class with less MP recovery options, and (B) a cast time, so you have to wait until after a big hit before casting it, which risks a healer of topping you off instead and wasting the MP, or you dying. If you try to pre-cast it for a tankbuster, it will be interrupted.