This one was a bit tricky simply because it's vague on where to do it. When you're on the "Front Gates of EEEEEEEVIL!" level, you'll be jumping along and Deadpool will start talking about Magneto's citadel. Once you land on the next solid platform, this is where you do this achievement. I found it easiest to use the enemies up and to the right a little bit. There will be some that do more damage to you. I melee killed everyone I could with the Sais, then picked off a few from a distance with guns to finish the combo. Same location as the "You've been Cole-slaw-tered!" achievement.
Video by WORLDEATER shows the location for this achievement: