Neither of these are necessary fights - completely optional if you follow the right steps - but I got REALLY frustrated trying to beat Kurt Zisa, and the Clock Tower Phantom in Kingdom Hearts. I didn't want to give up so easily. D:

Kurt seems simple enough at first, but it got really repetitive, really quickly. I got tired of having to go back and hit his three arms and then wait for him to stand up and then cast lightning like five times. Then as his health began to really drain, he kept sending sand tornadoes after me which were a pain to dodge. And then that spinning attack that stole like half my health. Bleh. THAT started to make me mad. I was so determined to beat him, but just gave up. =| 

I gave up on the Clock Tower Phantom even quicker. I couldn't even get THAT close to him or he'd start hitting me. And then when his stupid orb changed to show that I could hit HIM, he would just fly away from me. I also got tired of him casting doom on me. -__-

metroid prime 2 i think  
one of the bosses was like some underground worm or something? like the movie tremors. 
it was one of the earlier bosses.  
some giant digger thing and it was the bomb guardian or something. 

For those who have not played Halo 3 or Portal 2, highlight not the spoiler...
 

zubat and tentacool. every cave and every body of water in the game you find them. That's why its good to always be well stocked on Repel :-)

from pokemon games

I'd have to agree. Zubat's are so friggin abundant.

M. Bison from Super Street Fighter 2 Turbo.

 The dragon's in skyrim. They constantly get in the way of what I'm doing

1. Zubat from Pokemon because they are so annoying
2. Medusa Heads from Castlevania because once you get hit, you could possibly fall off the platform and get hit a bunch of time by them and die.

1. Zubat, Teantacool, and Blissey. Zubat and Tentacool are extremely annoying, and Blissey pretty much eats away at your soul until you can KO her.
2. Super Mutant Behemoths from Fallout 3. Sure, a mininuke can down them after 2 shots, but fighting them when you left your Fatman at home is a real pain.
3. Algol, from the soul caliber series. Hes THE most over powered character in the game, and its made worse when people have trained in his style online.
4. Behemoths from Final Fantasy, specifically IV and XIII-2. So powerful, so unkillable sometimes.
5. Frost Trolls. 'Nough said.

Also, I hate Reapers in the original Kid Icarus. They're not as unbearable in Uprising, but they take tons of hits to kill in the first one.

Husks from me3

You know those starfish enemies in metropolis zone, Sonic 2?

Actually, anything metropolis zone makes me shiver at the memory of the difficulty I faced racing through it.

Game : mega man: character:dr.wily

Rival
Pokémon FireRed/LeafGreen
Took me about an hour just to beat him when he was the champion!

Another annoying enemy is the Cloud of Darkness as the final boss in Final Fantasy 3. It was so annoying. I had to level up my team a lot just so I could beat it.

One enemy that annoys me to no end are the spinies in Super Mario Bros.

They aren't hard to avoid, but dammit, they just get in the way, and irritate me :|

WALLMASTERS!!!!!!!!!!!!!!!!!!!!!!!!!!!! They growl at you before lowering and they take you back to the beginning. Not TOO annoying, but they are my least favorite enemy. I'll leave it at that.

The droidekas in star wars battlefront, the first one. They never went down. If they didn't annihilate you with their guns it took all of your ammo to take their shields down.