First of it all, this is no story based achievement. You can miss it.
Ok. How i unlocked it:
I took the magnum and the crossbow and killed all enemy waves asap.
As soon as you can shot the energy ball with the gravitron you do it.
Then i shot at everything that moved and as soon as the gate was open i left. Right there the achievement popped.
Note: all resistance members survived. I think that is the factor here. At my other playtroughs i lost some.