Just finished the game again with Mechromancer this time, and it seemed much easier to kill him compared to when I was using Commando. I can think of a two main reasons for this:

1) I seemed to be taking less damage from enemies. Either I was taking hits from the Warrior better because I had an adaptive elemental resistance shield this time, or I was running around more instead of focusing on staying under my shield. Also, DeathTrap runs around looking for enemies. I think a lot of them (crystalisks etc) always focused on running after him instead of me.

2) I used a shotgun against the warrior. He takes reduced damage from all weapon types, but since he's so big, shotgun pellets him him - which negates the accuracy penalty that generally limits shotguns to close quarters. I was stuck draining my Assault Rifle with Commando, so the more time I spent killing Warrior, the more time it had to kill me.

So yeah.. when a DPS character doesn't take damage.. they kinda rule.

I finished off the final fight with Little Bit Trouble. I used Ordered Chaos for this one side-quest and found it really fun.. but I've heard that there seems to be a multiplayer bug that stuffs it up in multiplayer, so I stayed away from it. You also can't "take it slow" with that build since it's a shotgun build, which I had to do a few times when I didn't have suitable weapons.