Fun game but I gave up on the UFO boss. As someone pointed out, it's not really clear how the upgrades are supposed to help with it and whether they would make it harder. It just seems like something you can't out-upgrade (which goes against the idea of the game a bit) and I don't feel like I have much control over where the grenades are going.
EDIT: I beat it now but I don't feel like I did anything different, just had better luck with the grenade movement.