This was a really good idea and I especially liked discovering the dynamic between different characters, like how the small one is fast enough to evade the medium sized one and all that. The execution definitely could be worked on though. When the enemy takes damage it feels really random and hitting him three times in a row felt like an impossible feat. Slow the game down a bit (it could get faster as time goes on, but as it is it's super fast from the get go), make the interactions more consistent (the medium guys bullet very inconsistently overpowered the big guys) and I think this could be something really interesting.