how the hell do you get it. the description is too confusing.

Revive(or attempt to) the class that is needed for the primary objective over a class that isn't while near the primary objective, just as the achievement says.

Example:

You are playing Security in our favorite: Container City. At the door you have to blow up there is a Soldier and an Operative down. You are a medic, what do you do?

If you want the trophy/achievement you revive the Soldier, because the Soldier is required to complete the objective while the Operative is not. If it's near a hacking objective pick the operative, if it's near a repair objective, pick the engineer. I do not know if reviving an engineer near a damaged robot or a medic near a VIP will get you the achievement though.

ah alright thanks lol. =D