THE SHELL'S SECRET
The Shell's Secret is the most difficult level in the game movement wise, and can be a terrible reset. With this level, I would recommend only doing strats in runs that you are confident in. This is also the level where skill gaps are the most evident, there is a major time difference between the slowest and fastest tech.
Going up the cliff with wall jumps is very difficult and will probably require some practicing. I wouldn't recommend learning this right away. The most optimal way to get to the cliff is to waterslide on the spawn platform and roll out from the smaller platform (see example). Once you get between the walls do a sideflip toward the right wall. This sideflip, due to the angle of the walls, if you sideflip parallel to the wall in front of you, you will end up falling off the wall. In order to get the correct angle, sideflip slightly forward and toward the right wall (see below). Once at the top of the walls, you will have to buffer a spin jump to wall jump of of the next platform. Often times, you will have drifted a bit outward while jumping up and will need to change directions. In order to do this, spin the control stick in mid-air and hold in the direction you want to go. After Mario has been on the ground and changed his direction, press A
Groundpounding on ropes gives you an extra boost of height if you do another jump right after it. This means groundpounding into a jump dive is the fastest way to travel on ropes, however this is not needed in this case. Do two normal jumps holding forwad on the rope, and the seond jump should be enough height to ledgegrab the shell
Beginning Wall Jumps (Both)
For the first jump hold Up, then Down Right, then Up, then Down/Down Right. It is a lot easier than it looks
Using Groundpounds for Extra Height
When Mario groundpounds, he actually moves upward a small bit. Using this we can get onto the rope by the slope in the secret with a spinjump dive, which we would otherwise not be able to do. This also works with things like getting coins. If you are just below a coin hovering, you can groundpound and maybe reach it.
Triple Jump off the Slope (Hard)
To start the slide on the slope, hold the stick in the direction of the triple jump and press A. Do a buffered spin jump in the same direction then spam A when landing on the next platform (some prefer to just press A once, but spamming works better for me)
Glitchy Wall Kicks (GWKs)
Although not in my examples, GWKs can save you if you are short on your triple jump. To do a GWK, while sliding down the wall press Y and then as the camera is panning in press A
Strats for The Shell's Secret are named by the color of the peg on the first rotating block when Mario reaches it. ex: Green cycle, Red cycle. Sometimes they are also named for the peg color of the last rotating block before the shine.