I've searched everywhere, shot every wall, and everything. Plus, there would have to be about 110 more enemies in the level to get 100%. It is just some levels count the boss as an enemy and some do not, but why? And I don't even know how to explain the boss where you get 20%. So either there is a big secret, or the developers made the bosses different percentages for no apparent reason other than to annoy perfectionists like me.