This achievement requires you to add Wrex to your party and finish 5 missions with him.
You will be able to recruit him early in the game. After you land on the Citadel, continue the main mission until your investigation leads you to C-Sec. This is the second time you'll meet the krogan and your chance to add him to the party.
If you are new to the series, finish this achievement before Virmire story point.
Assignments counts as missions.