After 1-2, you should have the fire powerup. Since there are a lot of enemies, most of this room is done on the ground. The Skud enemy sitting on the block is in a position such that if you do two fires from the start, it'll land on you, so I take a little step after the first fire. In the next room, it's easiest just to fire up the stairs.
When going upwards, it's faster to jump on the platforms, instead of climbing from underneath them or floating up. In real time, it's hard to jump on all the platforms, so you would climb underneath one of them (left). In the next room, you combine for double cutter, and swallowing the powerup while falling through the loading zone saves a bit of time (middle).
There is a small window for using double cutter and not hitting the platform before getting Shard 1. Right before going through the loading zone, you can remove the powerup by putting it directly in your mouth. I like to face left when using the powerup, because if I use it too early, it takes a shorter amount of time to come back to Kirby for some reason.
After passing through the Adeleine room, there is Shard 2 behind you. To shortcut the whole room, you can float up near the Ghost Knight to go directly to the exit. Combine for double fire for the next two boss fights.
Dedede is straightforward, but note when he becomes vulnerable, using the health bar as a visual cue. There is an RNG-dependent strat which involves hoping Dedede jumps in the air with his hammer on the first phase, causing him to enter his second phase faster, but this is really unreliable.