Nope

2)is it pyramid head from silent hill 2?

Lets see...

1) Warcraft 1, he is in dungeon.
2) Bowser in Supermario
3) If I remember correctly, Cyberdaemon is at 4k HP. Rocket hit does around 100 damage, so around 40 direct hits. Still in Zandronum or similar, where there is proper mouse handling, most players prefer pistol or fists.
4) Castle Wolfenstein, aka the very first one top-down side-scroller before Wolf3d happened.
5) Eidolon from Hexen 2, to beat him its better to stay still in first form and constantly move in second form.

2) Kyle Katarn in Jedi Knight - Mysteries of the Sith
4) Doom II (it was a secret level)

2) Nope.
4) close but no cigar.

I don't know this one, but it sounds like a Metal Gear sort of boss for anyone that might have spent more time in that series. Or a fight where you only run / dodge and never actually attack the guy until he passes out or the environment takes care of him - hopefully that'll help someone!

It's El Oscuro from RoTT.

http://tvtropes.org/pmwiki/pmwiki.php/Main/HopelessBossFight

This is a whole trope (actually, a few tropes). I think guessing the particular example that you mean will take forever.

So, you'll åarticipate on the 5th giveaway I recon?

Almost positive GoG gave the remake away for free at one point. I have yet to fire it up - which is weird because I DO enjoy first person shooters and I don't buy too many of them so you'd thing when I get "in the mood" this would be an easy one to reach for.

Number 2 could be a lot of things.
Bowser from Super Mario Broz is like that too.
quick edit before ppl get pissed, i forgot about fireballs

2) Death? Or do you mean an actual named NPC in a game?

1)World of Warcraft
2)El Oscuro
3)40 rockets
4)Wolfenstein: Enemy Territory
5)The last serpent rider is eidolon and can only be defeated after
destroying the chaos sphere

This, probably the closest set of answers. Some nuances may apply.

1) World of Warcraft
2) El Oscuro. Rise of the Triad
3) The answer is either 40 or the classic magazine article that read "Protip: To defeat the Cyberdemon, shoot at it until it dies." (still have a copy around, I believe)
4) Beyond Castle Wolfenstein?
5) Eidolon was the last Rider to be destroyed (in Hexen II, requiring you to keep moving and shattering the Sphere), but D'Sparil was the last to be created (Heretic) as he is the youngest. Technically, they all still remain, as after the defeat of Praevus on the expansion they all disappeared together