Not a boss, but, I just finished Metro 2033(great game, btw. Currently writing a review for it).
*potential spoiler*
There is a part where you have to protect a npc from exploding rolling mutant balls that look suspiciously like metroids or whatever those ball things are(never really played Metroid in-depth). Problem is, the npc walks so as !@#$% and there are buttholes all over the floor that spawn the mutant balls. Serious,- they look like anuses. Anyway, so, the balls keep spawning over and over and over. Miller takes about 12 explosions before he is dead. I was playing on normal. I took 2 and I was dead.
I tried following him...kept getting a game over. Tried running way ahead to get him to run...nope, he still walks slow as !@#$% and dies. I retried for about 5 hours straight. Finally by luck, I got by that part. All my ammo was gone from shooting the balls before they could explode. Then guess what... another part came up. I was like "!@#$%....!@#$%..." About 20 game overs, then I discovered that the entire time you had to shoot the assholes that spawn the balls.
I was pissed.
Needless to say, after another 30 game overs(I had very very little ammo so i needed luck again), I finally got through it and eventually beat the game. I had to be at work about 2 hours later and hadn't slept the day before either.
It was worth it though
