You get this Achievement from completing the 6. Priority Mission: Tuchanka (Krogen DMZ/Aralakh)
Hint: If you cure the Genophage you get +650 Krogen War Support and Mordin will die, if you dont you also get the +650 Krogen Support + 175 Support from the Salarians. Dont make any other difference?
As GrmRpr69 mentioned: If you imported a ME1->2 Savegame (?) and then Wrex is in the ME3 and he to see through you Plan to not cure the Genophage and you get only the Support from the Salarians. If you start a new ME3 Game without a Import, Wrex is not in ME3.