This is level 63 of the Main tunnel.
In this level, if you want to use the Runner, use small, short hops. The size of the platforms are big, and most of the time the size of the gaps are not too large, so much of the level can be done through small hops. But in some parts of the level, the gaps will become large enough that you need long jumps in order to pass through the void.
In the Lizard's case, try to do long jumps every time you land on the platform. Jump high quickly, since the crumbling tiles crumble quickly, but don't panic and jump too early, since this can get you jump so far that you pass through the next platform in air, and end up landing in another gap. This can be threatening if you don't have enough skills.
That's why this level gains this difficulty.