Basically, what are some good places to get XP to get weapons to max proficiency level?
By killing guardians in last level? Or killing Rakk Hive?
If you have the DLCs (which I expect you do) going into Moxxi’s Underdome gives you a never-ending flood of enemies to kill for xp and to get your weapons proficiencies up and up.
In some ways it’s a little tedious since there is not a whole lot of variety, just waves and waves of the same type enemy in the same 4 arenas, but you can get your fill of kills.
I just run around Zombie Island. It’s what you might call a target rich environment. Either that or farm Claptraps at Hyperion Dump and the power plant.
The Larva Crab Worms in The Lost Cave. Fast runs, high experience, and super easy kills.
Switch weapons for the last bit of Craw’s health - that’s what I’ve always done.