Kaze and the Wild Masks

As the description says, you need to complete all islands without getting hit. It should go without saying, but this is the hardest achievement in the game. Two important things to note:
 

• This achievement requires all regular levels, all bonus levels, AND all boss levels
• The medals for this are unfortunately not tracked from the world selection screen (in the ship above the clouds). You will need to look at each level individually for the golden crown medal to see if you've done it

That being said, as I highly recommended in the roadmap, you should be going for these medals on every level before moving on to the next one. You want the level to be fresh in your mind to maximize the chances of success. Additionally, I highly recommend pairing these medals with the time trial medals on the majority of levels. This is because the time trial medals require you to beat the level without dying anyway, so you're most of the way toward this medal already. The other benefit from doing this is that, like with many good platformers that encourage speed, this game's levels are designed such that, if you're moving at full speed, obstacles and enemies line up perfectly so that you can keep going without issue. There are a few exceptions, but in general, after beating a level, you'll find that things line up pretty perfectly when going for the time trial. What's also nice about this is that, if you always go full speed, it keeps things consistent, and it will eliminate any "randomness" from your level playthrough, making it easier to see what mistake you made, fix it, and have it work properly every time after that.

I found most levels in the game to be pretty easy. The hardest of the regular levels are definitely the Dragon Mask levels, since they're autoscrollers. The last one in particular is pretty tough. As for the bonus levels, the first three aren't bad at all but the last one is really tough since it's just a constant bullet hell. Lastly, for bosses, it's just a matter of memorizing the attack patterns (there is no randomness to the boss fights) and responding accordingly. The first three bosses are pretty easy to learn and adapt to, but the final boss fight is really tough because it moves around in a homing pattern of sorts, meaning you'll need to move around to avoid the boss and can't rely on the exact same movements every time (unless you make the exact same movements each attempt, which isn't realistic).

The videos in Wait... What? (100G) show each level also being beaten with no damage (in addition to all collectibles). You can watch each to see them being done, but it's really going to come down to practice and muscle memory rather than a video guide. The only real places where the video guides can help is in the Dragon Mask levels if you need to check what's coming up next. That being said, feel free to use them as needed, I just found it better to play and practice each level, doing them quickly but safely to get the best layout for each obstacle and enemy.