It was great getting Crawmerax the Invincible, but I would like to see more hard boss fights on an epic scale. How about these?
1 Slimer the impenetrible.
A sycthid that is not overly large - think the size of a runner - but very fast and, as his name suggests, leaves a devastating corrosive slime wherever he crawls. Also vomits projectiles, flies, burrows through the ground to drag someone under, both injuring them and separating them from the group, and has a special skin that stops bullets in their tracks.
2 Pegasas the unstopable
A giant rakk with the special ability to pick people up and launch high and far into the air.
Thats all I can think of for now, add your own ideas below!