Not sure how that's possible not to get a single legendary after multiple playthroughs, 1) The Warrior Have has a pretty much guaranteed legendary per playthrough after killing him with the moonshot. 2) Killed Terramorphous? As in the "You. Will. Die" quest in the vanilla game? He always drops at least a class mod.
The list goes on.....
If you are talking killing the mega bosses then no....I can't complete them in SP as they murder me in seconds. The only way I have ever gotten weapons and items worth a damn is to the farm the gold chest. Again, I'm on playthrough 2, I've gone through all the DLC and at level 51 or 52 ( i think i said 62 before but I was wrong)
it really is that bad....though, I've enjoyed the game quite a lot but some parts were extremely difficult and tedious because the lack of really good weapons and especially with bad sheilds.