Try to keep four or fewer sprites on the screen at a time to avoid lag

Bosses will not be able to hurt you while they have iframes on. This allows strategies of jumping inside of the boss after you hit them to land on their head as soon as they become vulnerable. This is especially effective on the TNT boss at the end of Dry Erase Desert.

Keep in mind that projectiles can still harm you even if the boss has iframes on.