This and the achievements:
”It’s so realistic”
are all for collecting random drops from killing claptraps.
The quickest way to farm kills is to go to the final boss of Tartarus Station and shoot the claptraps that come out of the MINAC as they’re coming out. You can shoot the turrets first as they can be a pain.
Should take you a couple hours as the drop rates are incredibly low (especially for pizza), but keep at it and you’ll get it eventually.