A few tips for the shooter that spawned multiplayer mayhem.
You Are Not Alone: You're not the only one fighting aliens; the commandos are too. If you find yourself in a situation that has both aliens and commandos in the same area pick a nice hiding spot and watch them beat up on each other. When the fighting stops jump in and finish off the weakened winner. Backup: Use security guards as backup. Get up real close to them and press your "use" key until they say something to the effect of "You're right we'll have a better chance together." Once they're following you they'll engage aliens and commandos at will. Don't Camp: Commandos are very smart and once your position has been compromised it's on! Not only will they attack you in formation but they'll also lob grenades at you. So don't sit still. A veteran friend of mine once told me that the only way to survive an ambush is to run into it and the same holds true in Half-Life. Sure the odds will still suck but they'll suck less. Use Scientists: If you find yourself hurting and in desperate need of medical attention seek out a scientist. Walk up to him and press your "use" key until he says "Let me take care of that wound for you." This won't work every time but when it does it's a welcome surprise. Crates: Every crate or box you see may contain something you need so break all of them open with your crowbar (except of course the ones with explosives labels). Hazard Course: The Hazard Course is an excellent resource for learning the moves necessary to make it through the game. Even seasoned players should have a crack at it. Stepping Stones: As you will learn in the Hazard Course you can pull crates and boxes into positions so you can use them as tools to get to high places. Shotgun: The crazy scientist-aliens with the crustacean heads and spiny arms are dangerous yet weak and slow. Just one shell from the shotgun should do them in. Underwater: Switching to software mode should help you to see underwater. Be sure to switch back after you've reached dry ground. Security Guards: Your allies can still be of use even if you think you're done with them. When you no longer need a security guard around for example kill him and take his ammunition. But don't do so in front of another security guard: he will instantly become hostile and attack you. Electrocution: There is a monster in the power-up stage that is difficult to kill. Try turning on the track generator and waiting for the monster in the generator room. Once he's there run to the door and then back out the other side. This will electrocute him. Quick Health: Get health in a pinch by diving in the nearest water (not toxic waste) and staying under until the O2 warning flashes. Quickly surface and you'll get 10-15 health.