I think a good boss for "skill" would be Quadraxis
Here's the video of the Metroid Prime 2 Quadraxis Fight.
You can get the gist for size comparison and attack power with the video.
This boss fight in MP2 was epic, and it made you use just about all your powersuit abilitys.
In SSB4 tho, he could be a good challenge for using all your abilitys. Lets say he is a boss in the story mode. (It would be possible for more than one Quadraxis to exist. In the light temple of the Sactuary Fortress, some peices for a Quadraxis was part of the scenery, proving this might mave been produced by the Luminoth more than once.) Be it Subspace particles, the Ing, or something else, a Quadraxis is somehow activated. The characters you use can be anyone and the location you fight it can be anywhere.
The battlefield itself would be as big as Bridge of Eldin, if not bigger even bigger. Quadraxis would be in the center, and the ground would be completely flat with no holes and with raised platforms to the far left and far right of the battlefield.
The battle would consist of multiple parts too.
Part 1:
Starting with the opening cutsceen, and then the battle Part 1. Part 1 would be to bust Quadraxis's leg joints. Dodging will play a big role in his attacks.
Location status: Quadraxis would have 2 legs that are on the battlefield and 2 in the background. The joints are the only damageable parts of Quadraxis, so attacking the lower part of the leg or the feet are useless and will result in knockbacks with 4-10% damage. The only way to get the other side of the stage is to jump over Quadraxis's legs or go under his feet. You wont be able to walk under Quadraxis's feet untill you bust the orb inside it. Then just walk under his feet normaly. Be warery tho, his foot orb will respawn after 20 seconds.
Vulnerability Status: Using the platforms to your advantage, you must jump and hit the leg joints with attacks. However, the joints can only be hit when the joint rims are flashing.When 4 joints remain, There will always be 2 joints flashing at all times, but the select 2 will not always be the ones on the battlefield. The change rate for the flashing joints to change is every 8 seconds, but each flashing joint is set off 4 seconds from the other flashing joint, so change rates are different. Attacking a non-flashing joint will be the same as running into a leg. When 3 joints remain, There will still be 2 always flashing, same change times. When 2 joints remain, only 1 will flash, but will still change every 8 seconds. When 1 joint remains, it will cycle by itself, flashing for 8 seconds then not flashing for 8 seconds. When all joints are destroyed, the Part 2 battle will begin.
Movement status: Quadraxis does not move left or right at all during Part 1, but it will occasionaly rotate it's legs clockwise or counter-clockwise, so 1 leg moves to the background and 1 comes onto the battlefield from the background. He does not attack while rotating, but the leg stomps can hurt you. The stomps will be in set locations: Where the legs are supposed to be and in the middle of the battlefield, so a leg rotation takes 2 steps for Quadraxis. If a leg hits you, it will cause will cause medium knockbacks and 8-14% damage. If you get crushed by a foot and the foot's orb is still intact, You will plowed into the ground as well and the orb will shatter. However if the foot does not have the orb, it will just act as being his by the leg itself. Try not to move under it's feet untill it has stopped rotating.
Attacks status: Here is his arsenel and how he attacks. Most can be easily dodged. There will be a second sometimes where he will not attack, like all bosses do.
Note: All attacks originate from the center of the head module, which is in the background, so beam attacks dont trace the whole battlefield when fired, just when it crosses planes with the battlefield.
Annihilator beam: Quadraxis fires three shots of the Annihilator beam. Each shot is fired 3/4 seconds apart from the last one. The beams can chase you to an extent, but can be out-manuvered. Being hit causes 5% damage for each hit and small knockbacks. Will KO around 120% damage.
Charged Annihilator beam: Quadraxis will beam 3 red lasers from it's 3 eyes, which will track you down. The lasers will come together and pin point you. This takes about 5 seconds, however when it fires the laser will remain straight where you were located, so the shot will not follow you, but the path of the laser. Then it fires a charged beam that moves rocket fast! The beam will cause 20% damage and a heavy knockback. Will KO around 70% damage.
Minigun: Quadraxis fires a rapid-fire minigun. Each beam will cause 1% damage and just flinching, but the attack fires 20 beams. Takes about 3 seconds to perform, and wont KO.
Jump shockwave: Quadraxis jumps off the ground and lands with a big smash. Being hit by the legs causes normal moving leg damage, but if the foot's orb is intact it will cause a electrical shockwave that will creep along the ground it both directions of the foot. The radius in the middle of Quadraxis is about from leg to leg, then the same distacne on the opposite direction of the leg. Can be easily jumped over, but contact will cause around 15% damage and a medium knockback. Will KO around 100% damage. Note: background shockwaves count too, so look out when the shockwave crosses planes with the battlefield.
Cyclone spin: This is the most dangerous and rare attack Quadraxis does. Quadraxis will move move foward untill his head is above the battlefeild, then it will put its legs together in the middle and start spinning around rapidly. This causes the player to be suctioned in to it, unless the player can outrun it. Dont worry, if you are a slow character Like Bowser or Ganondorf, the suction power will be just the right amount as that you can run away while still moving foward away from Quadraxis slightly. Careful with fast characters tho, you dont want to move too fast and run off screen. Jumping is a bad idea while Quadraxis is spinning. If you are in the air, the suction power will bring you to Quadraxis instantly. If it is able to pull you in and hit you, it will cause a rocket knockback and around 35% damage! This attack can KO you easily around 40% damage. Quadraxis will try to pull you in 7 seconds untill it gives up and returns to it's normal position
I will make Part 2 later. Ive been typing for awhile
