To defeat Cupid you can use many different methods. One way is with "Spring shoes" the other is with "Icarus wings" You will find Cupid on the early levels of the game. Be watchful of his arrows they will make your controls change and not for the better.
Method one: Icarus wings, Fly over Cupid with your Icarus wings and the Cupid will fall and pop.
Method two: Spring shoes, line up the Cupid and hold the x button down for the biggest jump possible. If your aim is good you should fly over the Cupid and he will fall and pop.
Method three:Hi Top's, This will take some good timing and you will need to be near an edge (water/space edge). While running you can run towards an edge and leap a great distance and pretty high. You will need to time your jump so when you leap, you will jump though or over a Cupid. (Thanks Leeono)
Untested methods: There are two other ways to kill mobs, and can kill Cupid. I cannot say if the achievement will pop with either of these methods. I will need to test them to verify.
Untested one: Tomato rain, You will need to open the present "Tomato rain". This will drop tomatoes like random raindrops doing damage to anything they hit. If enough of them hit Cupid he will drop and pop.
Untested two: Viking opera singer, paying the singer 3 bucks will cause her to sing a really high note and kill all mobs on the screen. If Cupid happens to be one of them he too will pop.