This is very simple all you have to do is first kill of the turret. Just throw 1-3 JB genades the turret dosen't shut down throw another one (be careful of imlumiati soldiers just turn mutant for them). Thats the turret down! Next refill health and mutagen and shoot one shot of the messon cannon to weaken it then shoot it with parasites (the shield is weak to parasite for some reason.) Be careful of iluminati soldiers remember. To finish it off (the sheild should be red before trying this). Get your BBG and finsh of the sield and gray!

On the mission"The Greys" Get to the fourth room There is a computer activate it and it will move the train that you start in go back to the start am go across the track to the end of the track and walk in to the blue warper that warps you to the shark room

Well it's not a cheat per say but here some advise to kill super theta, get your favorite gun a blast him, when he charges jump or beast mode it and move change back and shoot, when he jumps up to let the phanto ball things wait till there all on the ground then gernade them and they all die instantly, when you think you shot him enough turn into beast mode and shoot him with those little thing that come out of his hands..
(sorry can't think of it)

Although melee attacking isn't as fun as loading an infected body with bullets it does save ammo if you really need it in the future.