The Cyberdemon is the first boss in the game. You fight him at the end of Mission 9 (Lazarus Labs). The fight is fairly easy. In the first phase, he mostly fires rockets which you can easily dodge by running in circles around him. When he uses the laser beam, just do a double jump to the side. A great weapon to down him quickly is the Heavy Assault Rifle with the Micro Missiles Mod. The missiles inflict a lot of damage quickly. When his health is depleted, he will respawn again with a full health bar. He will now be throwing some projectiles at you. Do one jump (not a double jump) to dodge them. There is also a checkpoint at the start of the second phase. When the second phase starts, immediately use the BFG to take away a lot of his health. Then, use the Heavy Assault Rifle or Plasma Rifle to kill him while dodging the projectiles.
Defeating the Hell Guards
The Hell Guards is the second Boss in the game. You fight them at the end of Mission 11: Necropolis. The fight consists of two phases. In the first phase, you fight only one of the Hell Guards. He has a shield and you can only damage him when the shield is down. Use a weapon that deals a lot of damage with each hit. The Gauss Cannon with Precision Bolt mod is recommended. Try to get headshots while using it. When he jumps at you, there will be some fire on the ground -- so make sure you jump when he does to evade the attack. Basically, all attacks the Hell Guards do can be evaded by double jumping (in both phases of the fight). In the second phase, there are two Hell Guards, but they do not use shields -- so you can kill them very quickly. When the second phase starts, use the BFG on the two enemies. This will take away a large portion of their health. The Heavy Assault Rifle with Micro Missiles mod deals a lot of damage. It also causes the enemies to drop health and ammo. Simply run around them in big circles and shoot rockets at them to quickly kill them.
Dopefish reference
The Dopefish can be found as the logo on boxes of ramen noodles from "Dopechan Noodles" found in various areas.
Fallout series reference
At the start the game in the Prologue where you wake up and leave the area he was kept in, look at the bottom of the security door. You can see that a Mixom Manufacturing sticker has partially slid down, revealing that it was covering a Vault-Tec logo.
Skyrim reference
In Mission 6, search the area near where you first encounter the Baron of Hell in the Kadingir Sanctum to find a cave with a hidden lever and a skeleton. The skeleton is wearing a Dovahkiin horned helmet and was shot in the knee with an arrow.
Terminator reference
Jump into lava in the "Foundry" mission to sink to your death while giving the thumbs up.
Defeating the Cyberdemon...