Sleep-Skating is an achievement in Run 3.
(Note: Since you can only earn this achievement with the Skater, it will be the only suggested character.)
This achievement is very similar to Isn't That Cheating? and uses similar gameplay. The trick is to move to either the right or left so that you are right next to the adjacent side next to the side you are currently on . And just like Level 8, there will always be a side for you to stand on. However, there's more to this achievement.
As the requirements say, there's not moving sideways at all, unlike Isn't That Cheating?, so you can't use the exact same strategy. At first, you may be perplexed, but there is a way to complete this achievement using creativity. If you don't understand why the same strategy for Isn't That Cheating? won't work on this level, here's the logic behind it. When you re-spawn after selecting Level 7 on the galaxy map, you will find that your starting point is in the middle of the beginning platform, in which case you would have to move to the side to that location where there will always be a platform to catch you. This would break the rules of not moving sideways as said in the requirements. So how can you complete this achievement?
If you haven't figured it out by now, the way to complete this achievement is to start by playing Level 6 and then play Level 7. During that small interval of time between the end of Level 6 and the beginning of Level 7, align yourself next to one of the sides and you're done! However, not all of the 6 places where you are in the middle of two sides in the tunnel will lead to success (remember that each of the 6 routes have a different number of adjacent crumbling tiles and gaps). Experiment with different routes in Level 6 and different edges to run next to in Level 7, and you should find a route that will work.