Fracture

Using the Bangalore (the Alliance rocket launcher), accumulate 50 kills.

Note: Gun kills apparently stack even after death or loading from a checkpoint. If you find a good point in which you are killing many enemies with this gun and you've just saved, press start and select "Load Last Checkpoint" (or whatever it is). You'll reload from the last checkpoint but the game's internal counter doesn't. Thus, the amount of kills you had with this gun prior to your reload will not return to the point of that save.

You can earn this achievement in "Weapons Training" mode, which unlocks after you have found 10 data cells. Equip the weapon, then stand in one of the red, pulsating circles to summon enemies. There is a red circle downhill from the building you spawn in, and another on top of the tower to the right.
Some of the weapons don't unlock until you have found a set number of data cells, but this particular weapon is available either as soon as you have unlocked Weapons Training mode, or at worst when you have found less than 30 data cells.
I think (but I'm not sure) that kills in the campaign are included in the count during Weapons Training so you won't have to kill 50 enemies in this mode to get the achievement.