To obtain this achievement you'll first need to be playing as Axton and have reached level 5, as using the Sabre Turret is his unique Action Skill. Now that you're playing as him, all you'll need to do is use his Turret on 100 enemies making sure they die by it. There's a few Skills worth buying to make the process go faster though. So, feel free to level up and spend some Skill Points on them.
Below are the Skills that may help;
Guerrilla Skill Tree
Sentry: Increase the number of shots your Sabre Turret fires in each burst. Also increases your Sabre Turrets duration.
Required Level: 5
Skill Tier: 1
Skill Upgrade: 5/5
Scorched Earth: Adds multiple rocket pods to your Sabre Turret.
Required Level: 15
Skill Tier: 3
Skill Upgrade: 1/1
All you need now is enough enemies to use the Action Skill on. To start, this may be easier to get playing solo because then you don't have to worry about other people stealing much need kills.