I have a bunch of unfinished challenges like "Black Betty" which require me to kill large numbers of things with specific guns/grenades/shield effects. Just going back to low-leveled places makes this "easy" but I'm wondering where the best place(s) to do this are, since all areas have a limited number of enemies after all.
Where should I go to farm lots of easy kills for challenges? Assume I'm level 50 and can go back to playthrough 1 to kill enemies of any level with little effort.
Locations differ, but I can not think of any that is undoubtedly better. I'd suggest just picking a location and finding a fastest route through it.
To consider:
I'd personally pick Caustic Caverns or Tundra Express. Both have many Varkids (I am grinding shotgun-in-the-face on them :-), plus Tundra also has many easily-headshottable bandits.
My Favorite is tundra express varkid area :D as Orc JMR said.
Reason for choosing varkid area. Varkid comes in swarms :) so more enemies at a time very good for nova blast challenges. They have very little health so killing them will be easy. Just kill them save and quit the relaod and kill them again :)
Vault of the Warrior, there areVolcanic Rakkthere that seems to spawn endlessly charging at you. Just stand at the top of the steps that leads to the end of the area. So at least there, you have a steady stream of enemies to kill (one every 3-5 seconds or so). Not to mention there are shops nearby so you can replenish ammo. So with the right weapons, you might be able to grind out your challenges without doing too much work.