I have a "killed Merasmus Lvl 20" precisely because of what hobbit said above.

I went in to trade, started shooting, and the mod (Who had a Valve Scattergun and noclip) almost instakilled the Lvl. 20 Merasmus. I had just hit him, so I "killed him" so to speak.

Now, with legal means, I've killed a level 5 Merasmus.