Those

Ambush Commanders in Borderlands 2!
They have this big ol' shield that makes them practically immune to any attack from the front. Now, dealing with their non-boss equiveilants, Bad...Elite Nomads, are no biggie. Just walk around them, despite their large frame they usually don't have that much health. But, due to to the arena you fight them in, the fact that they spawn with their backs to the walls, their high health and shields, and the never ending swarm of psychos and maruaders, the last one being really deadly on TVHM and above, make them really annoying to deal with. Oh, did I mention there's 4?
The only good thing about them is that once you kill them, they don't come back, and thank the Lord almighty for that, otherwise Sawtooth Cauldron would be more of a pain than it already is.
And if you can't tell, I'm still salty from when they destroyed me in my first playthrough.