Sea

Level Ups:

Mario [6]: ATK (after star)
Mario [7]: SP (after star)
Mario [8]: SP (after star)
Bowser [9]: ATK (after star)
Geno [8]: HP (after star)
Mallow (will be showing Peach's sprite) [7]: HP (after star)
Peach (will be showing Mallow's sprite) [10]: HP (after star)

You will need to perform the Swap Glitch.

If you aren't comfortable with this technique, or if you aren't comfortable with the movement to get all the Starslaps, either head into the room left of the Shaman salesman and make a safety save; or, if you only miss one, get into an encounter with it and use a Fire Bomb (and then depend on freebies to do the remaining Fire Bomb battles in the route properly).

Otherwise, head into the right room.

(video credit: Mill)

Get underneath the Star, jump, and immediately press X during your jump.

This star gives 8 EXP per hit.

(video credit: Justin-credible)

If you miss one, you can fight it and toss a fire bomb to fix your EXP, but it's honestly better to just reset and try again if you can. Bombs are quite scarce from this point onward.

Head toward the Sunken Ship.

SEASIDE TOWN | SEA | SUNKEN SHIP