Pretty straight forward, play and finish a "Fun" Level. I used level 27 for this achievement. Start by entering the following code: RY48 9BFY 6?JM KBK? in the passcode menu to start the level. Immediately grab the rocket launcher and fire away at the arachnotrons on the pillars and do not stop moving. Once these are dead a mega sphere will appear, once picked up a cyberdemon will need to be killed in order to continue. Once dealt with an army of mancubi will appear, clear them out to finish the level.
Here is a video of the level and how to beat it.