Ok I can confirm that you can do this by level select :D
So, Load up Genosha on Genetically Superior (Easiest Difficulty) change your weapons straight away to hammer and pulse rifle just fight through the enemies using the hammers, Kill the flying guy with the pulse rifle, about 5 minutes through the level you will see a cheerleader looking girl with I LOVE DP on her shirt :P as soon as you get to the stage press RB to skip cut scene and POP achievement unlocked well done :D
PS. I am not a deadpool fan so I don't know all the marvel characters as I am not really into comics although I did like the film Kick-ass :)
Important. To people who have been using the pulse rifle alot, if you are to run out of ammo quickly before you start fighting the enemies. Go to the menu and select settings and put autochange gun off (this will allow you to kill enemies with the pulse rifle without worrying about a weapon swap when the rifle is outta ammo :)
Edit: nemesis646 has confirmed you can use grandes thanks for confirming this :)