This achievement is unlocked through normal story progression and cannot be missed. It is the final achievement in the DLC.
After determining the source of the leviathan's transmissions, you travel to a planet to confront it. You will travel underwater and eventually a cutscene will play. The achievement will unlock after this, as the Normandy is loading.
The events prior to the underwater scene may be hard on higher difficulties, as it takes longer to get the charge to 100% and brutes will keep spawning if you let it fall too much. I recommend you keep moving and watch your health so that you don't get cornered.