Unless you are trying for the "Level 17 or less" ending, level up...a lot...now. Fight every enemy to clean sweep the tunnels, re-enter Bowser's body, re-exit His Unconsciousness, and repeat. Levels 15-18 for both Marios is my recommendation.

If you are trying for the 17 and under ending, only 1 of the Marios must be at or under that level. Luigi is recommended to Level Up. Get an "A" Rank in Jump Helmet and keep the item you get on Mario (Daredevil Boots, Double Damage on all attacks but getting hit (even for 0 damage) or jumping on a non-jump-able foe instantly KOs you, cannot be prevented except through dodging).

Bowser's Moves - How to Dodge
>Winds up for a Punch - Jump after ~1 second to dodge, ~1.2 seconds to counter
>Snorts then winds up for a Punch - Stand completely still and he'll hop over you
>Goomba Storm - Jump the running Goombas (keep your fingers ready for the flaming ones dropping in after the runners are done), ready both Hammers, and release the targetted Mario Bro.'s Hammer until it ends
>Fire Breath - First time will have dialogue where he says something like "Now I'm gonna burn ya!" then Starlow tells you to Jump onto her and hold on (Starlow will circle around the Bros., Jump and hold the button for each Bro. until the flames stop)
>Shyguy Squad - Ready both Hammers and release both when His Grumpiness rolls close, alternate Bros. for 8 strikes, speeding up each time.