Your initial assault on Dark Matter's fortress has resulted in most of its minions being wiped out. Most of what's left are some mini-bosses you've fought before, but they are all in a weakened state from your previous encounters. An innocent angel has been imprisoned in the castle and stripped of her ability to fly. Recover all of her feathers to obtain the final Heart Star!
Door to door
The stage starts off on an intense note, as the screen immediately scrolls upward and you have to contend with a ton of Nruffs dashing around on multiple levels. As you near the top, there will be three doors. The left door lets you get Rick, the middle Coo and the right door contains Nago. After that, you have a choice of another three doors. The left door contains a Maxim Tomato, the middle one contains a 1-Up and the right one only has a star. There is yet another set of doors after this. The left leads to Kine, the middle Chuchu and the right Pitch. After that is the final set of three doors, but only the left door can be passed through, the others are all Nidoo. You will end up in a room full of Sparkies. Grab their Spark ability and proceed to the next room. Use your ability to break through the blocks to get a feather, and don't forget to grab the energy drink too. The feather is required to get this stage's Heart Star.
Heart Star: Revenge of the bosses
This level almost entirely consists of boss battles, as you will face every single mini-boss you've defeated before. The first battle is with a Blocky. Its charge attack is as easy to counter as ever and it seems to have significantly less health. Replace your Spark ability with the Stone ability once you defeat Blocky, as you can use it to obtain another feather in the following room. There is another energy drink here in case you're wounded from the last fight.
Next up, you will face off against a Jumpershoot. Jumpershoot keeps all of its attacks, but the smaller space makes it harder to evade its assault. However, it does appear to have greatly reduced health similar to Blocky, so a few attacks should be able to dispatch it with ease. Get the Parasol ability to get the feather, then get the energy drink nearby to heal up. It should be obvious by now what the routine of this last level is. You simply have to defeat a room's enemies, take their ability, use the ability to get a feather, then continue with the next boss. The bosses you will face after Jumpershoot are Yuki, three Sir Kibble, Haboki, Boboo, and Captain Stitch. After the marathon that is the previous level, this final level actually seems quick and easy in comparison. If you've collected all of the feathers, you will get the Heart Star from the angel at the end of the level.