This strategy had me get this first time on Hard, after several cheap defeats with me just trying to figure it out.
To summarise:
Make your way to just outside the boss room and equip the two secret charms, one from beating the game, and the other from destroying all 25 statues. Then head inside.
Once the fight starts put on the makeup kit, and the garter for a sweet 10% damage boost.
After that stick to three hit combos (X, X, X) ending with a Dragon Feet special (hold B). Keep moving, away and return - rinse repeat. If you get the opportunity to stamp on them, especially if they're on the floor together, go for it.
Avoid the blonde haired chick's fiery torpedo attack at all costs if you're playing on Hard (just keep moving, don't wander horizontally in line with her unnecessarily) as it absolutely decimates your health.
Eventually they join hands in the middle and start spinning around one another bouncing in the air. Just sprint (double tap direction) left and right underneath them. After 4 bounces, they stop and become stunned. Tag one with your 3-hit combo+special, then move over and tag the other.
In the third phase they'll bust out a new move where they join hands in the middle, and start rollerballing on top of one another around the arena chucking out heart shaped projectiles. Just go to the bottom right corner, and walk up the screen - jumping over the boss when they hit the top right corner and head down towards you. Then return to the bottom right hand corner, rinse repeat until they stop.
In the final phase, one of the girls gets brought back as a ghost that floats around above the action, whilst the other roams around continuing to try attacking you. Just keep jumping, and attacking the ghost when it stops moving. The hit box is pretty generous - I found jumping in to the ghost from a diagonal particularly effective.
You can make use of a 2nd controller if you have one (especially if you're going for the co-op & secret character achievements), and put your other character in to the fight in the last phase and you will be largely left alone whilst the other girl goes after your ally - leaving you free to concentrate on the ghost.