I'm trying to spare every boss, is there a way to lower enemy morale besides attacking?
This is situational, actually! Some enemies can't be spared, and it's up to you to figure that out. This mechanic is somewhat diminished in MERCANOID, as it's largely a prototype for my framework.
There are two bosses you can spare, IIRC; the Slammia, and the first phase of the final boss, with the latter having a specific mechanic to drop Morale.