Because there are not yet cheats available for Half-Life 2 we introduce you with this little HL2 Guide. We strongly doubt that cheats that any other website show are correct since the game is not yet released.

Headcrabs:
Danger Level: Low
Their Attack: Lunges at your head. If it grasps your head, it will drain your health until you are dead.
How To Kill: If attached to your head, you can only use a crowbar to kill it. If on the ground, I recommend saving ammo and using the crowbar.

Headcrab Zombie:
Danger Level: Low
Their Attack: Swipe at you with their claws. Turns into Headcrab when defeated.
How To Kill: If all you have is a crowbar, walk behind it and strike at it so that it cannot hit back. Otherwise just shoot it with a gun and then beat the headcrab with the crowbar.

Combine Soldier: Danger Level: Medium
Their Attack: They lob grenades randomly and shoot a machine gun.
How To Kill: If there are 2 or more together in one spot, drop a grenade. Other wise jump and fire at them. If you can get an angle where they cannot shoot you but you can shoot them, take it. Alot of the times its best to sit back and watch the ones that lob grenades kill their allies on accident. Less work for you in the end.

Flying Sawblades: Danger Level: Medium
Their Attack: Fly back and forth buzzing like a sawblade.
How To Kill: You cannot kill them. However you can use them to your advantage. If you have a Manipulator Gun, you can pick them up in mid air and shoot them into a path adjacent with an enemy. If done right, the Sawblade will kill your enemy for you. You can also get around them by hitting them with your crowbar to repell them away from you.

Flying Manitee Thingy:
Danger Level: High
Their Attack: Flies back and forth shooting energy bolts down at you.
How To Kill: Best way to defeat this miniboss is to constantly drive your buggy forwards and backwards so that when it fires, the bolts will be where you were a second ago as you are on your way to the other side of the screen. Other than that, just keep firing the gun at it and dodge the bolts.

Strider:
Danger Level: High
Their Attack: Shoots energy bolts at you.
How To Kill: When you first enter the Striders screen, do not go to the bottom. Instead, stand on the other side of the first ammo chest, across the laser fences. Be careful not to touch the laser fences on either side of the screen as they mean instant death. While standing next to the crate, pick up all the power ups you can until your health and HEV suit are at 100. When you have achieved this, drop down to the bottom level and the Strider will appear. Hop back up to where you were just standing, and when the Strider stands fully erect (haha) throw all of your grenades at it. About 10 grenades will knock it down to 50% health. After this, continually jump up and down to evade its energy blasts while firing your Machine Gun at it. If it moves to stand on top of you, go to the other side of the screen and do the same thing on the opposite ledge. Continue to jump and shoot and follow these steps and you should be able to defeat it pretty easy.