Tip for the medal about getting all hearts:
Most are rather easy to find, but there's one behind a fake wall in the castle, in the long horizontal corridor with the multiple spike balls. Look at the walls, one doesn't have a torch, that's the entrance to the last heart.
I looked throughoutly the first playthrough and didn't find it, had to read through dozens of comments here to learn about this.
The medal pops up right when you collect your last heart, so as long as it doesn't pop, you're missing one. If you beat the boss you have to start all over, no continue from last checkpoint before that.
Don't bother with the daredevil medal, since the attack button doesn't register all hits to 100% it's luck based to win against the boss, even if you figure out its attack pattern. Died on the last ping-pong section 2 tries in a row because of that, and since there's no checkpoint before the boss, you'll have to run all the way through the castle again, with just 3 hearts. Not worth it in my opinion.
