Devil walker is easy. All you have to do is crouch on the right side of the tunnel that is directly to the right side of the devil walker and shoot at its legs. It's cannon hits above the other end of the tunnel and it's energy rockets hit far enough away from you to do no damage. If you run out of ammo run in a tight circle for 10-15 seconds and you'll get refilled enough to take more shots (it's random how many rounds you get and for what weapons your carrying) and if enemies start shooting at you. Just dispatch them real quick and continue shooting the devil walker from the spot I mentioned above. Barely even get touched