This level has many tricks. First, the key can be tongued through the wall through use of the rock. When Yoshi jumps up into the rock, it forces him left or right. If Yoshi's tongue is out when this happens, the tongue will be forced through the wall, allowing Yoshi to tongue the key. The positioning for this is the most difficult part: in general, the right side of the rock should be just covering the keyhole, and Yoshi should be standing as shown in the picture below:
The best way to set this up is to push the rock to the left, then jump over the rock and go to the right side of the bottom wooden platform. You then push the rock over to where it is just covering the keyhole.
You the go underneath the rock and get the key. The inputs for this are press y, then b a few frames later and hold both down until the key has been moved closer to Yoshi. Then, walk a little bit to the left (pushing the rock) and retongue the key in order to tongue it through the wall. The second section can be skipped by egging away enough foam to extended flutter onto and stand on. Then, a perfect flutter through the hole the key falls out of allows Yoshi to continue through the door to the end of the level.
When clearing out the foam, it's possible to shoot past the flower cloud and hit the piece of foam above the screen guarding the key, as seen in the IL. This saves you the time of jumping into the piece of foam to clear it, saves a half second from avoiding collecting the flower, and eliminates the risk of a bonus game. This shot requires the flower cloud to be further to the left in its oscillation when you shoot, which depends on exactly how you pound through the big column of foam previously. Try to pound through the left part of the foam to get a good cloud spawn.
The key can be reached without using the rock by carefully positioning Yoshi's tongue through the wall with a little bit of momentum, true swag~ To get the key, run alongside the bottom and tongue then jump three frames later. Hold left after jumping to adjust the way the key flies through the air.
Once the key is touching the wall stand back far and tongue once more to get the key. Standing too close will cause the key to bounce against the wall and fly back. If the key is barely too far away to get this way, try tonguing from the left edge of the platform with a bit of rightward momentum. A well executed rockless can save 8 seconds compared to a perfect rock attempt.
Rockless in action