To obtain this achievement you'll first need to be playing as Maya and have reached level 5, as Phaselocking is her unique Action Skill. Now that you're playing as her, all you'll need to do is use her Phaselock on 100 enemies. Again, there's a few Skills which may help it go a little quicker, so feel free to level up and spend Skill Points on them.
Below are the Skills that may help;
Motion Skill Tree
Converge: Your Phaselock ability now also pulls nearby enemies towards the original target.
Required Level: 15
Skill Tier: 3
Skill Upgrade: 1/1
Quicken: Increases the cooldown rate of your Phaselock's ability.
Required Level: 20
Skill Tier: 4
Skill Upgrade: 3/5
All you need now is enough enemies or simply one 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 making kills before you get to use it. Remember that the enemy(ies) do not have to die while in the Phaselock.