its really not that hard..
just a tip, the bit where whats her face A.I says something like "well look at that, looks like they are trying to keep us out of our own base" and prior to that you just saved a bunch of marine foot soldiers. Well as soon as you save the foot soldiers dont select them just as quickly as you can move your warthogs up on the icy patch infront of the sheild barrier thing is a troop of about 10 grunts use the ram ability and kill them before the A.I controlled troops kill them. Make sure you dont have your foot soldiers selected otherwise your warthogs will move at snail pace. Just make sure whenever there are A.I troops attacking grunts you focus on killing the grunts before they do.
I did this and had about 7 grunts to spare (the ones in the top right corner of the base) so i could easily fetch the skull and then kill them for mission complete.
Also just save at the broken bridge before you reach any foot soldiers incase you stuff it up and dont kill enough grunts instead of doing the mission over and over again
I did this on normal and got it about 3rd try once i realised the A.I were stealing my kills
Hope this helps!