Mind Flayer from Demon's Souls, particularly the Black Phantom version.

Puff a stab to the back will do the trick now for this one :P

You cant even fight it properly

Medusa Heads are incredibly annoying, especially when they're located in areas with spikes (since being hurt in stone form causes double damage... and falling into spikes already deals a lot of pain).

Sea Creepers from Resident Evil: Revelations are annoying as well (unless you have shock grenades). Nothing worse than two Sea Creepers in the same area without a Shock Grenade. Prepare to mash the Y button.

Chilfos from The Legend of Zelda: Twilight Princess. That room full of them in Yeto's mansion was terrible.

Is there friendly fire in random team games in Halo?

friendly fire is a big important part of halo and it's great because people can't just shoot with the tank in a group of players from both teams or throwing a grenade to some people and only kill the enemies which would reduce the needed skill level of the game and you have to take much more care for your mates but it's obviously not so great when your teammate kills you on purpose.

so, halo is much better with friendly fire but those guys are bad for halo^^

Anyone else ever rounded a corner straight into a Deathclaw and said something very, very rude?

it has to be malboro from final fantasies.

I never found these guys annoying.

the fleet in Faster Than Light.

Yeah, elites are actually way more annoying in the first game. Their shields are horribly overpowered and their movements are very fast