You must be at least a level 5 Siren to unlock and be able to perform Phaselock. Once you have it unlocked, you can use the ability any time it is charged by pressing LB when facing an enemy. You currently cannot do the attack without an enemy in range.
I used the ability every chance that I could during fights and unlocked the achievement during the quest "The Road to Sanctuary." Of course, this will vary based on how much you fight/the order you complete quests. Point is, it took me roughly 3-4 hours to get it by casually playing (I wasn't even aware of the achievement). In the video, I show it unlocking for me (for anyone who is curious).