Painkiller Hell Damnation

In order to get this achievement, I used a combination of the shotgun and the rocket launcher/minigun. With the shotgun, I used the secondary fire option to freeze enemies and then unload a shell in their face. They popped every time. The rocket launcher is perfect for groups of enemies but ammo can get a little scarce if you are careless. With the rocket launcher method you should also get the Five Points Gangsta for killing five enemies in one rocket shot.
On Daydream difficulty this achievement is a cakewalk and should pop about half way through. Good luck and I hope this helps.