Thanks a lot for your answers !
While making the game, I planned to add a "death" animation to the enemies, in a way similar to what you explained, Somebody. But I lacked time and I chose to improve gameplay over graphics.
However, I didn't thought about the sine movement for the player and I will give it a try. ;)
I will continue to improve the game, I still have a couple of things in mind to do. Anyway, if you enjoyed playing it, that's the most important. ^^