Damage on bosses is not more important than damage on trash packs. Everything in the dungeon needs to die (unless it can be skipped, but opportunities like that are rare) for the dungeon to be complete, so it's actually more efficient to save mana at the end of a boss in order to Moulinet the trash mobs more, since you generate the same amount of mana either way and Moulinet gets more damage out of whatever mana you generate.

My earlier post suggests a scenario that allows a third finisher to exist without rendering the existing two obsolete. Remember that overuse of those mana tools is a potency loss, and at some point the loss is so big that it's better to just use a normal finisher.

I'll admit I was being facetious with the last remark quoted, though only because I thought the question was also facetious.