In the battlefield mode,pull out a grenade,cook it while walking up to your enemy(best to do it from behind),get a little tiny bit close and stand there.wait ford it to explode.(really good in a crowded area).
On the 1st level. Helghast Assault after you run away from the tank and you are in that little bunker fighting off the Helghasts if you look behind you. Right at the back of the bunker you will see a Helghast Mask with a tube on it but no Helghast.
Okay, most ppl with gameshark already knows this, but for ppl who just got it, activate infinite ammo and no reload time, and when you start as ISA, if you hit secondary fire butto, BAM, auto fire grenades that never run out, also watch that you don't hit yourself with explosions. Also works with gun that fires missles as secondary
If your playing on the battle field remember this, always reload after every fire fight, the sniper rifle kills in one shot(you don't need a head shot) and the mini gun comes with a grenade lancher(the grenades are a bit like rockets, always on thier own and are grey)
You might already knows this but some people dont.