You must be pretty dedicated to playing Half-Life to be this good, and it's nice to see that computer gaming is still a major source of fun for many. Anyway, here you are, probably have beaten the game on nearly all levels of difficulty, but still want to see what other people suggest as strategies. Well for one thing, Weapon usage is major. Conservation of ammo and use of all weapons is the game plan here. Since there isn't one majhor topic to discuss here, becasue at this time you should have your own style of play and strategies, I'll get to the tips.
-As I said, conservation of ammo is key. Short bursts from your automatic weapons and precisely aimed headshots will preove to be invaluable.
-Ammo conservation also comes into play when you skip over secrets or ammo even in plain sight. Search every nook and cranny for extra supplies, especially on the higher levels of difficulty where enemies take unreal amounts of hits to kill.
-The rocket launcher is laser guided. I didn't relaize this until I fired a rocket and turned around to go for cover and looked at a wall right next to me. Anyway, since it is laser guided, you can pull off all sorts of tricks with it. For example, you can aim it around corners by firing it and then when it approched the turn, quikly pulling the laser left or right. Experiment with it.
-Another weapon with a little secret is the guass gun or tau cannon or the one that shoots orange lasers. When you charge and fire it, the gun has mucho recoil, so try aiming at the ground and jumping right before you fire it. In the air you go, ontop of a large building or a ledge.
-Use all your weapons. I kicked myself many times when I ended up with no ammo for my conventional weapons when I could have thrown a grenade or two. I don't know why, but it seems that people are afraid to use the snarks, the grenades and the remote mines.
-Enemies are AI; and the AI is not perfect. They run on a "node grid". That is a collection of specified points where they will stop for cover, stand, or pass through while seeking their enemy. This can really come in handy for you. If you are battleing a marine across the hall and he keeps going from behind a crate for cover to a point outside the crate, he will most likely visit that same point again. So find cover that just barely lets you aim at that point and wait for him to move there. Placing a remote bomb on an enemy's node might work, too.
-Especially in the Xeno levels, try a tactic I refer to as waiting and killing. You walk into a room only to find massive groups of enemies. Run back into your doorway and lure each enemy to you one at a time. THis doesn't work all the time, as they try to work together, but it does work sometimes. Another twist to this is to get behind cover and slowly move out of it to the side, stoping to shoot any enemy that comes into view.
-During the big standoffs between the marines and the aliens, don't draw attention to yourself until the end of the battle. That way, more enemies will have killed eachother before focusing on you.
-The crossbow is extremely powerful. This is taken for granted all the time. If you can, try to snipe a few marines before closing in.
-Another gun that is often overlooked is the revolver. It is extremely powerful, too.
That's all the advanced tips I can think of today. If you want to, send in your own tips and I will credit you with it and all that good stuff.