Agreed, prismantus is very easy unless you have a terrible team. The amount of players I've been with that cant handle the fact of who goes on top of who (hurrhurr) and decide to just end it all but taking all 3 of us off the cliff.

The hardest besides the one we mentioned would be the clone Triforce Heroes at the end of the den of trials.

Agreed. Went through whole den o' trials and got to that boss, and lucky me, Blue Had To Leave. Whoopty doo.

You're telling me? Bro I had to do Den of Trials alone since I was bored and the servers where being updated or something, imagine that.

Rafaam on Heroic mode...worst hearthstone experience of my life

I usualy do the game on single player, too. Made it to fortified zone on single player. Did he rest on multi player. Then blue left

-Most bosses in any Donkey Kong Country game.
-This isn't really a boss, but, that insane button mashing part in star fox adventures for gamecube.

This is probly really late and will seem like fanboying (because it damn well is and I'm ok with that).

For context, Mythic is the highest difficulty in WoW raids and dungeons for Warlords of Dreanor. I've personally raided up to the fourth boss in Mythic difficulty Hellfire Citadel. The entire team has to be on point, or the entire team will fail because of one players shortcomings regardless of their gear. We had a couple people who were severely undergeared but still performed well enough to help us get the first kill on bosses.

The following video is the world first kill for the current Mythic difficulty "final boss" of the Warlords of Dreanor expansion, Archimonde. Keep in mind that a good portion of the abilities in the video flying acrossed the screen at any time would oneshot anything but the most prepared of groups. Method is considered the grouping of the best PvE players in WoW.

They wiped 472 times before they got the kill. Let's just say that the final phase of the fight is absolute bonkers.

http://www.youtube.com/watch?v=Vx6ipbVOWvY

Some of the boss battles in Xenoblade Chronicles 3D was hard for me. One of them was Mumkhar

Another boss I had difficulty on was that Jenova Birth I think? From FF7

Lil hunter from Nuclear Throne, He ambushes you on 5-3 and he is annoying as ****.

All of the dragons in Dragon Age Inquisition unless you're a higher level than it and have better equipment.

Knuckle master from a link between worlds.

Mine were inity from pokemon colosseum lvl 100 doubles, and hunter x from kh2 on proud mode.