Dude, those were so dumb! I don't even want to talk about it... but I will for the sake of a fellow achievement hunter...
It involved me and my then girlfriend being very determined and leaving her Xbox on all night. If I remember right, we'd destroy all the dangerous guns, then hide behind a safe rock, and leave it running all night, then in the morning we'd wake up and run around looting. I think we did this over the course of three nights over two different weekends before we finally got everything we needed.
I am not sure which was worse/stupider, the Mad Moxxi achievements, the Claptrap achievements, or me and my GF deciding that we needed to get them.
Incidentally, I have only one achievement on Dead Island that I have not managed to get. The stupid orange weapons achievement (loot five orange (super rare) weapons). I need one more, but they are so rare I've only managed to find four out of five in nearly three play throughs, so who knows how long I'd have to play the game to just find one more. It is one of those achievements some people seem to get in a single play through without any effort or thought put into it, and others have to spend hours and hours and hours of effort to get.