Right, so first up, that level's a bugger. Personally, I went for Phantom Ganon first because he's the easiest, then Ganon, and then the Imprisoned. Took me a while, using Master Sword Link, Dual Blades Ganondorf and Linkle/Volga (idr which, I think Linkle).
Phantom Ganon's irritating but fairly easy, first round is Dead Man's Volley with the white ball or just hit all five small black ones back at him. Helps to have some range before doing this, he also floats up a bit higher before either of these attacks iirc. Then, you need to hit his back, although the blast radius from throwing bombs at his front will usually hit his back too.
Ganon's just old boss strats again. If he starts sticking his hands into the floor, Boomerang. This phase happens twice. Right hand summoning a fireball, bombs. Left arm has an eye and fires lasers, Bow. Starts flying and breathing fire, Hookshot him when his tail glows. Then, either just after his rush attack or when he sticks his hands in the floor, shoot the glowing crystal on his head (Locking on usually guarantees the hit). For all of these attacks, it helps if you're a bit back from him, being near means he'll just spin on the spot a bit. Bit harder because he's in a corridor iirc, but it's doable. Just try not to get the Imprisoned attacking you too.
Imprisoned's the worst of the bunch. Save him for last so you have special attacks and fairy uses, because you can utterly whack his toes in one shot with a fairy or most special moves.
Otherwise, normal rules on mini-boss rush. Even if you knowingly screw up on one of the criteria, finish it anyway because Ganon and Phantom Ganon drops are buggers to find normally. If two or more are attacking you at once, reset. You'll be pushed to beat the challenge at all with two of them (usually Ganon and The Imprisoned) attacking you.
Also, other news bit I didn't say earlier because doublepost: Medli and the Master Wind Waker pack are out on the 19th May. AKA this coming Thursday. Although it looks like all Medli's weapons and stuff are in the MWW map, so there's the inevitable moneysink.