Aqua 2
This is the first level with significant amounts of moving water. It came into play in 2-3, but very minorly.

As in the previous level, combine two powerups (stone and fire) without swallowing the first one. The difference here is that the two enemies are found in different rooms. Right before the Bobo, you can pass over the Emp with a few well-timed jumps.

The miniboss fight is straightforward with the volcano powerup. Conveniently, the first attack always shoots up, so use that to break the block. Collect Shard 1 after the fight and remove the powerup as you leave. Note that you can't remove a powerup in the next room.

In the river autoscroller, there are a few places where you can jump that save some time. The first place is where the Kany (crab) is, saving about a third of a second. In the middle of the autoscroller is a jump for Shard 2. Finally, jumping before the penultimate waterfall saves about half a second.

At the beginning of this next section, inhale for a short amount of time to avoid faceplanting. Then, swallow the Kany enemy, grab the invincibility candy, and just run forward. Shard 3 is at the bottom of a set of two waterfalls. Besides avoiding faceplanting with the enemy in your mouth (allowing you to just hold right), you can also spit at the enemy after your candy wears out.

Since the water is running to left, jump repeatedly up the slopes, headbutting Bouncys when convenient. When you get underneath the Bronto Burt before the Galbo, jump and inhale to grab fire while landing on the platform. Make your way to the second Galbo by alternating jumping and fire. Here, float up and single fire across to the platform near the top of the waterfall. Perhaps the most time-consuming mistake is hitting one of the Splinters (spiky logs). Usually, these logs hit multiple times, causing your recovery to take forever and possibly knocking out your fire powerup.