Spoilers
This achievement is for finishing the quest of getting the female krogan who is immune to the genophage. You get this quest after taking in wrex. It is part of the main story so you can't really miss it. Once you talk to the salarian council and wrex and victus on your ship, wrex will want you to find a cure for the genophage before the Morgan's ally themselves with the turians in the fight against the reapers.
If you killed Wrex in the first game don't worry. Another krogan named Wreav (thanks ScuzzyBunny) will be there instead.
This is apart of the main quest so you can't miss it.