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Name: Dark Samus, Esteemed One (name given by the Space Pirates), Dark Hunter (also given by the Space Pirates), Metroid Prime
Weakness: She has her limits. If someone hits her enough, or mutilates her body to the point where she WOULD die, she will. While difficult to defeat, she is far from invincible. The lack of time to recoup in this 'game' will hinder her somewhat.
submit to me. submit to the inevitable
If attacked with too much Phazon, it will overload her; it's a double edged sword. While it does weaken her, the Phazon burst from her hits in a larger radius.
Advantages: She is cool, calculated, and will not let one failure stop her. Her lack of compassion, while cruel, enables her to use her allies and even those loyal to her. She doesn't care if they die as long as she gets her way.
everything I you desire will become a reality
Prime's wide array of abilities are useful for many situations, even with their weakened state. If given time, some troops, and she's got a playing field, she can work something out. Her sheer strength and her body's durability combined with that terrifies the galaxies before her.
What game are they from: The Metroid Prime series. (Specifically Metroid Prime 2 and Metroid Prime 3)
Attacks: In her own universe, her attacks are similar to Samus', except for the fact it's all Phazon. The Morph Ball has a slightly different appearance, but the functionality remains the same. She can also make temporary clones of herself (known as 'Dark Echoes').
She's able to use Phazon to possess the bodies of anyone she wishes; there were times when she had the Space Pirates at her beck and call, and other bounty hunters fell prey to her. Once the body overloads via Phazon, they become puppets to her.
even if they weaken me, they cannot stop me
As an assistant trophy, her powers have been weakened to a point, as has Phazon itself. Being overloaded with Phazon is incredibly unpleasant, and it will leave lingering effects after a match before it fades. However, since Dark Samus leaves the battle before too much time passes, it is not known if it has any lingering effects if Dark Samus is allowed to exert her will.
phazon will overwhelm each and every galaxy
Dark Samus' attacks have been limited, and her missiles were taken away for the safety of the fighters. Her Phazon burst is almost useless in a wider area, though it is better in smaller corners.
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In Zelda, there are really 3 or 4 "main bosses" Gannondorf being endgame (mostly) and Vaati being endgame as well. Demise is basically Gannon. Now, there is Bellum, but not a lot of people have played Phantom Hourglass, and he only appears there, but that can also be said for Majora (although skull kid appears in 2 other games). That being said, there are also side bosses, like Zant, Girahim, Yuga, and etc.So I believe that with these bosses there might be 2 or 3 zelda bosses, being there are so many. Since we do already have people playing Majora and Zant, I think a good nomination at least would be for Gannondorf.
Name: Gannondorf
Weakness: Light
Advantages: Darkness
What game are they from: Basically all the zelda games minus minish cap, Four swords adventures/four swords, Phantom Hourglass, and Majora's mask.
Attacks: uhhh his standard attacks like in smash??? idk i dont know what this means
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Name: Culex

Weakness: No known weaknesses, but once all 4 elemental crystals are smashed he is left open for all the attacking. However, he does become wide open for physical attacks when he's alone, taking extra damage from them.

Advantages: Has 4 elemental crystals that act as his weapons and shields. Has the ability to heal himself. Never backs down from a challenge, so scaring or intimidating him won't work towards victory. He himself is highly resistant to magic, but easily hurt after the 4 crystals are shattered and get's hit physically

What game are they from: Super Mario RPG: Legend of The Seven Stars

Attacks: Use of the 4 elemental crystals in front of him. They contain the powers of Fire, Storm (Wind), Water (Ice, etc.) and Earth. When all 4 crystals are shattered, he uses space itself as his weapon. Raining down meteors, charged beams of energy, and the power of his Dark Star, or uses a direct body attack with his hands or his entire body as his weapon when he runs out of magic to use. One signature move called "Shredder" removes any added abilities to his opponents. Wether they were added on by magic or from a magic object, they become negated (I know this sounds really OP but it's not as bad you might think. He's just really strong and hard to beat XD but not invincible by any means, I'll be sure to make that clear)

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I know this vid is kinda long > <" but this might help you get a better understanding of my description. Hope you enjoy it! Give me feedback if I should keep this boss or use someone less complicated or over-powered looking. Please let me know!
Youtube

(thinking on going solo, Pittoo acts this way all the time XD)

Name: Twinbellows the Ferocious
Weakness: i think weakness would be the water attacks since this dog is a fire element, he's slow in speed.
Advantages: Two Heads, double the problem.His heads can attack on both sides. Fire elemental attacks he's immune to and is strong around mountain terrains.
What game are they from: Kid Icarus, Kid Icarus Uprising
Attacks: Swipe, Charge , Fire Breath
Picture: http://www.kidicaruswiki.org/images/f/f6/Twinbellows_Uprising.jpg

* some info on him also - http://www.kidicaruswiki.org/Twinbellows_the_Ferocious *

OOH! I really like the sound of that idea! =D a two team kind of boss! And no worries about not knowing Mario as much as you do Fire Emblem ^^ I don't know much about Fire Emblem myself XD sadly ;w; This is just based off of my knowledge, and we could brainstorm perhaps a smaller but still large enough boss to be considered for at least one or two people to take down instead! Big ideas, but just starting smaller before we really get into things X ) I love and thank you for such an input!

Name: Medeus
Weakness: Falchion, any dragon-slaying weapon, a divinestone
Advantages: He is the last of the Earth dragons, and he despises humans for mistreating his kind. His hatred for humans makes him beyond reason, so negotiation is futile. Darkness in people's hearts sustain him, so people with impure hearts should not fight him.
What game are they from: Fire Emblem: Shadow Dragon
Attacks: Gah, I forgot! I gotta play Shadow Dragon again. I think he uses a dark earth-like power with his Earthstone.
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hey lets use pit's boss tbh i love both of them but i love dragons