There are 11 marathon modes that you need to X rank: 6 are new worlds, 5 are legacy worlds. There is also a really easy bonus world that you will need to X rank ONLY if you want the last character skin, but it is not required for the achievement.
An X rank has the following criteria:
- All levels 3 starred in a row
- No deaths
- No resets
The 300 stars requirement is clearly a bit redundant, as you need all 300 to unlock the legacy marathons anyways. I've included a video of every world with decently repeatable and quick strategies for X ranks :)