Holy water > all
Yes, even against a flying bat. During the boss fight, immediately get the upgrade that's in the right block of the lower platform. After that stay behind the remaining block, left or right. Wait until it flies towards you and when it flies near the platform throw holy water against the block non-stop. The boss should stay still when it's in contact with the burning liquid until it dies. Holy water freezes enemies for awhile, it works even better at higher levels. You can kill every boss except Count Dracula this way, easy peasy. You should try to get two upgrades in your subweapon as the trick works best that way. Or bosses might break from it.
And Jesus, stay away from the Caps Lock, my eyes hurt.