For this achievement you need to phaselock 100 enemies. This can be done with the Siren and using just one enemy and luckily SS Bricktop's guide for the 360 version still works. This achievement will unlock naturally if you choose the Siren for your playthrough, but if you picked a different class then this is the quickest way to get it.

This can be done as soon as you unlock the Siren's special which you do once you hit level 5. Play through the intro and select the optional quest called Shielded Favors. This quest grants access to the Souther Shelf - Bay area. Chances are you hit level 5 right by the time you get to this area; if not, turn back and kill some more enemies (not in Southern Shelf- Bay though, we need the Bullymongs over there later on).

Once you have the special unlocked you can go to Souther Shelf - Bay. Don't go too deep into the area, just far enough to agro the Bullymongs that are close to the entrance. Lure them back into to cave and position yourself