Just some general pointers really;
With the Spider Tank encounter, its easier to sprint down the slope with the knife equipped (movement speed is increased slightly). If you make it down alive, try and find a position from which you can shoot the Spider Tank without being hit yourself. There are a few spots behind the barricade that let you do this.
I found knifing to be extremely efficient against raptors, seeing how your invulnerable to melee dmg whilst in a knifing animation. Maybe its just me, but I found using the knife the easiest way to kill the buggers.
Other tips - TAKE COVER! Use the flare on the shotgun, it helps - a lot (plus there is an achievement related to the use of the shotgun's flares). And master the sniper rifle. On this mode, this is the gun to have, as your not having to deal with any "breathing" or crap like that, and a well placed shot will 1-hit-kill most enemies in the game. It's also got two stages of zoom, which is a very nice touch.
There are also some points in the game where you'll find having a minigun with a full clip of ammo to be a god-send. I'm mostly thinking of the room towards end-game that is filled with loads of flying insect things. You'll know it when you get to it! Placing a minigun down as a turret here can be a life saver. Also, specifically for this part of the game, use your knife! I've said it before - it increases your movement speed ever so slightly, and that added boost can help you get your ass out of dangerous situations that tiny bit quicker.
So good luck and play hard!