Yeah, I'm not a fan of raid bosses either, I prefer to either solo or just co-op through the game. Unfortunately to get certain challenges or achievements, you'll need to kill them. I suspect I'll call on the help of a couple of level 50 mates to assist when the time arrives.
Btw, I presume when I do beat the game on playthrough 1 and get handed the YOU WILL DIE mission from Tannis for Terramorphous (level 52), I can just have a level 50 mate join my game, kill the thresher, and then I can turn it in? And repeat that on playthrough 2?
I was playing solo as my assassin last night (running down the entire right skill tree to ultimately get the achievement for 10 seconds of deception) and ran across the Black Queen in Bug Gulch. A few shotgun blasts later (handy having a decent amp shield AND a shotgun that launches fireballs of death) and she fell, dropping a 9000 damage orange rocket launcher as a prize. I say rocket launcher, it operates more as a grenade launcher! I was on my way to complete the "Good, Bad, Mordecai" mission, so I thought I'd try it out in the Mexican shootout. Two shots later, and both enemies were dead. Colour me impressed. I thought I'd see how it fared against Old Slappy next. He didn't like it one bit, and died after one shot as well!
I think I might be keeping this rocket launcher for a few levels just yet...!