Yeah, I recently just finished the game again after 100%-ing my first file.
In my first file which was my first run, I just levelled up attack after realizing that Bowser's shells drastically increase his defense points. Since every move is literally avoidable and depends on your reaction time, I didn't do too much with HP. So I just put it into attack and special
With the brothers I did it a bit more randomly, still keeping special points in mind. It definitely helps to spam Might Meteor and that Spring jump move, it helps for a lot of damage.
Broggy Bonker works wonders against one-shotting Dark Bowser (I mean it forces Fawful to revive Bowser) the only downside is that you have to go to every area and look for Blitties. It's definitely worth it though
And Flayl is right, all of the random encounters will go by faster. Look into getting beans also once you get that drill bros. power-up. The attack and sp beans help a lot also. one point can make or break you being able to do a third special move, or the difference between killing your opponent, you know.
I think I ended up at level 42 for all of them in the first file. Of course the beans jacked up my stats, but whatever.
In my second file I purposely avoided everything, so I think I was like..level 15. Speaking of which, being above level 40 or below level 17 gives you two different messages at the end of the game aside from the normal one.