The Sabre turret is Axton's skill and is deployed by pressing
once it is charged up.
In order to deploy the turret you need to level Axton up to at least level 5, then at every opportunity you can deploy the turret and have it lay waste at your enemies. Enemy level, size, type ect does not matter so long as they are enemies, so long as the final hit is the kill shot by the turret it will count. The turret can turn 360 degrees, so its best to place in the centre of a group of enemies.
Note: If you have unlocked this on the 360 and transferred your save and downloaded it on the One simply have the turret kill one enemy and this will pop.*Also, thanks to wolfzero01 for the heads up: Don't use the "...To Pay the Bills" challenge as a way of tracking it.*
That challenge seems to be for getting kills *while* your skill is active, not just *with* the skill. If you kill an enemy with your own gun while the turret is deployed, that will add one kill towards the challenge so it will inflate the number. It's an odd distinction but it's enough to mess up the tracking.