Explodemon

I got this in the second fight with him in world one. As the trophies says, just defeat him without getting hit. If you do get hit, you can restart from last checkpoint (which will be the start of the fight) and try again. I would recommend going for it on the earlier levels because he is much easier so this shouldn't pose much difficulty.

If you don't get it by the time you finish the game, then go back to world 1-2. He will start in the middle - jump towards him and jump away and he will begin to pull everything towards him. After this, get to the top platform and knock the purple explosion crates into him below. It should only take a few hits then jump down, use your aim and blast away. Repeat this and you shouldn't have much trouble beating him without getting hit. You can also just knock the purple crates into him if you want. If you do this fast enough he won't even have time to do any damaging moves to you.