When there are no enemies around, carefully press Y (or L if you have Gravity Gun) until the turret stands up straight and is activated. Pick it up with your Gravity Gun or use key, and face up while you have the turret and it will re-activate. To re-activate a turret, crouch and face the way the legs are sticking out from the turret. Also, make sure that the object is indestructible, and most importantly, look at the enemy and see if you can kill it with one shot.
Note: Make sure there are no more that one enemy while doing this. Then if possible, shoot the barrel at the enemy. When you need to run a long way and you have some objects with you (for example, a non-explosive barrel), if you have the Gravity Gun, use it as cover and run (while facing the enemy) to the other side. Getting past turrets and other dangerous enemies While playing the game, press Y, B, A, X, White, Y, X, A, B, White to spawn an ammunition cartridge for your current weapon.
From: and Refill ammunition for current weapon While playing the game, press Up(2), Down(2), Left, Right, Left, Right, B, A to spawn a medpak that will restore 25 points of health. While playing the game, press Left(4), Black, Right(4), White to unlock all levels.
