That stage uses line clear mode, which is described in-game (and I figure you know how it works, since level 38 is also line clear mode), so I'll refrain from explaining the basics. Now, unlike the previous level, which simply wanted you to clear 3 lines total, this one is asking you to clear 2 or more lines simultaneously (which yields more points in this mode). Theoretically, you could clear every line on the screen simultaneously, but I was never that cruel anywhere in the adventure mode.
Edit: To clarify, this is easier than it sounds because lines don't clear until your combo is over, so the lines don't have to be formed by the same CHAIN, but rather can be formed by the same combo.
Good luck!
