Start by killing the two black ops already there. Then throw grenades as fast as possible while moving around as much as possible to avoid getting too many hits. After throwing 3 or 4 grenades turn mutant. Hit (press R3) the black ops to kill them and to gain mutant power then direct your shots (press R1) at the grey. Don't have to be too acurate since the things that come out of the hands once you're mutant, kinda know which way to go. Just aim as much as possible towards the grey because if not they will go towards any black ops near by. It may take a few tries. Stay calm! Once mutant you can move a lot faster thus you're able to dodge most bullets....go the matrix!

If you aim 4 the aliens head they will die faster, and doing that you keep most of ure ammo, til you find more. And their is plenty of ammo scattered around the place anyway.

In order to break the grays shield you'll need to have a lot of grenades and very powerful guns and skill, first use all the grenades you have then use all the most powerful weapons you have and while your doing this black ops will be attacking you non-stop , Grays produce them over and over again.