Half-Life/Nihilanth

Welcome to the final boss.

Nihilanth has two attacks that will deal a lot of damage:
 * Attacks
 * 1) Shoots purple balls at you. They do some serious damage so be ready to dodge.
 * 2) Teleports you to another room where you will have to fight other creatures and make your way to a portal to get back into Nihalinth's lair.

Nihilanth begins immune to all damage. However, he has a weakness. There are three external power sources (yellow crystals on the wall) that must be destroyed before Nihilanth's head will periodically open up. This will leave his brain exposed to any of your weapons.
 * Weakness

Environment
You will meet Nihilanth face to face on a rock. However, like in previous levels, this platform will give way and you will fall down. The gravity is drastically reduced here so jumping is quite high.

You also have alien trampolines. They are needed to get up high enough to shoot at Nihilanth's brain and to reach the crystals. But be careful as some of the trampolines will shoot you quite high and could end with you falling to your death. To slow your falls down, try to slide down the walls.

There is also a platform with a trampoline and health pool. This pool is risky to use, given that Nihilanth has a clear shot to you when he floats up.

Teleport Rooms
Nihilanth will shoot a green orb periodically. If this orb hits you, you will be ported into one of 4 rooms where you will be forced to fight off various creatures and head to the port to take you back to Nihilanth's lair. They will port you in this order. There is one good advantage to being ported though, The rooms have ample amount of supplies. So if you need to restock, this is a good place to end up.

Room 1
The first room will find you falling down a long narrow pit. Xen masters will assault you when you land. After killing the xen masters, make your way to the top of the pit by jumping from platform to platform. There are other dead scientists with supplies and health packs that you can grab on your way up. The gravity is ridiculously low so you can jump from one side of the pit to the other, but this also makes guiding yourself to the next platform rather difficult. You will not die by falling so don't worry about this. When you make it to the top, jump into the portal and start fighting again.

Room 2
This Room is similar to the first one. you will not die by falling and gravity is reduced. You will also be attacked by xen masters upon landing, and there are some decent supplies in the corner. After killing the xen masters and gathering supplies, jump on the red rocket-rocks as they land. They are incredibly small so some presision work is needed. Once you land stand still until it take you up to the portal and jump though the portal.

Room 3
If you are unfortunate enough to land in this room, you will be happy to know that you won't be ambushed by Xen Masters. Rather you will find a bunch of vortigaunts. After killing them you can at your risk jump into the pool for supplies. But you will also find the big fish so do so at your own risk. One good thing about this place is a health pool on a ledge. Otherwise, make your way up to the top and head through the portal.

Room 4
This room is the same as the last room, except instead of fighting alien slaves, you are up against a gargantuar (or other large monster, but this was my experience).

Strategies
The following sub-sections are various tactics and strategies that have worked for different players.

Old fashion shoot at him method
The old fashion way to do this is to jump on the trampolines and shoot his brain. This is great if you have strong weapons, but is difficult to do. There are a few other methods people have tried that have worked.

Skullcrusher method
Recommended by many because if your jumping is successful, this is the safest and fastest way to win. Some people have managed to jump into Nihilanth's brain and blast him. This is quite effective if you can end up here. However, it is difficult to jump in here. If you make it, you will not have to worry about getting shot or ported here (EDIT: Nihilanth may be able to shoot you/port you inside in Half-Life: Source). However, no explosives here, as you will also end up being blown up. Shotgun, and your power weapons (other than rocket launcher) are best here. You also don't have to wait for him to open his head.

Jumpshot method
If you have grenades, this is also effective. Just jump on the trampolines and throw grenades in his head like a basketball. His moving higher makes this easier as well. If you have particularly good aim, you could use the grenade rounds from the assault rifle as well. However any other explosives, (remote mines, proximity mines) will not work, thus limiting your range. This is very effective as far as damage is concerned, but you also have to be careful not to die by falling, or bolts. You will also be subjected to portals here as well.

Hide and shoot
If you purposely go into all three rooms that you are transported to through Nithalinth's portals, you will be able to collect ammo and then not have to worry about dodging the portals later on. Now destroy all three crystals (crossbow works best, only two bolts per crystal) and he will now take damage. You will now only have to avoid the electricity he shoots. Keep shooting from the cover of a spike (will take only a minute or two, green slime will show it is working) until his head opens. When this happens, he will only be able to shoot one ball of electricity at a time, so you will be much safer. Bounce up and fire explosives, e.g. from the MP5, into his open brain. It can be done in only one bounce up if you fire grenades quickly enough. This may be much easier than the "Jump shot method", as although both of these strategies are essentially the same, there is no need to avoid portals, as they will be cleared and most of the work is done from cover.

Ending
After killing Nihilanth, you will see bolts of electricity shooting from him as he shakes the world. Now make a new save file. You will then be ported and come face to face with G-Man himself. You now have two choices: work with G-Man, or don't, in which case you will be put in "a battle you have no chance of winning...Rather an anticlimax, after what you just survived...". If you approach him you will get the good ending, and if you don't, you will receive the bad ending - The "battle you have no chance of winning" as mentioned by G-Man. If you made the save file, you can return and watch the other ending.

After the ending, enjoy the cool music and feel proud you beat the game.