To get a sequence of 30 or more, the objective is to try to put any pieces that don't have a "breaker" (the ones that remove pieces of the same color) immediately down practically anywhere, and then use the breaker pieces with more care to try to keep your multiplier going (which is dubbed the sequence). Use the B button to place down pieces immediately. You have 5 seconds between multiplier increases to break another chain. When in doubt, you can just spam the Y button to use all your items (if you are getting closer to the end) and hope that it causes some kind of chain reaction.
You can do this on Endless or Mission mode.
Here is a video of me getting a Sequence of 31: