Ok this might be a stupid question but I'm confussed :p .. I'm at the part..

This guy's hard. Basically, follow the light coming in and use the mirror shield to shine it on him until he solidifies. Then pick him up using the power bracelets and chuck him at a point on the wall with spikes going all the way up it. This will make him explode into all the smaller solid ghosts. Kill as many of them as possible before they reform. Then carry on until all the ghosts are killed, where at that point, the ghost skull thing will try to fly off by dies in the light.

First, go stand in one of the areas with light, then use your Mirror Shield to reflect and shine the light on Jahalla (the boss' name). Continue to do that until it goes solid, then go pick the Ghost up with A and throw it at the spiked walls with A. When Jahalla hits the wall, it will split into a bunch of
Poes, now destroy as many as you can and then repeat this. When you defeat all of the Poes, you'll beat it! (Hint: if you have the Spin Attack from Orca - use it!)

Go under one of the lights that are shining down and use your mirror shield to reflect the light onto the boss, then after he turns visible, throw him into the spikes around the room. He will hit them and then explode, then just use your sword to kill as many of the little versions of the boss as possible. After you kill all of the little versions, the boss will die.