The best place to farm this is probably the very first time you team up with Gaige and Deathtrap. Just let Deathtrap do all the work (set the game to easiest possible) and just before he finishes off the last enemy, save and quit and re-enter the area. You can do this in any area that has Deathtrap, but that first encounter may be the best spot for it.
If you do happen to have already cleared the DLC, then there's two nice things:
1) There is a single mission (Empowered Grawn) that Deathtrap will always spawn into to help (as he is required). Sadly, this can be a slog of a wait, especially whenever Deathtrap focuses on the boss involved, but at least you aren't locked out of the achievement with this character.
2) The number of kills is tied to a character, like a hidden score, but NOT to the playthrough. This means the number is carried across Normal and True Vault Hunter modes, so if you've cleared the DLC on one mode you can play through the other mode and farm kills there as well (of course, TVHM is tougher, but hey).
Of course, this means that if you switch to a different character/save file, they have their own tracked number of Deathtrap kills so you can't rack them up by swapping characters.