You have to watch the marines, twice they destroyed it on me, so here's a few hints.
-Destroy AA Wraith and Ghosts
-Kill all marines when necessary
-Get at least 2 of one type of grenade, grenade jumping was easier for me.
-Be sure to have a sniper and Plasma pistol, to immobilise the wraith, ahd kill Turreter.
That's all I got for now.
Credit to .Impact for the sig & avy