Cheers for the feedback! 😊 The UFO boss (along with all bosses) has two versions, if you're on the normal version it is a case of jumping up or falling through a platform after shooting it whilst in it's path. If you're on the AMPED version then it is a combination of that, but also if it is using one of the additional attack patterns indicated by an arrow you can sometimes get a shot in from behind if you're on the level it is going to move to.
I'm glad you like the music, Virgill is nothing short of a magician with what he can do with Amiga MODs!