What's the highest you've ever hit? I've gotten close to 800 several times. I'm pretty sure 900 is possible with optimal equips. To the best of my knowledge that would be this:
Weapon: Lazy Shell
Armor: Super Suit
Acc.: Attack Scarf (or would the Quartz Charm work better?)
And that would have to be on a level 30 Mario who's gotten
only attack/defense bonuses for each level up.
The best method I can think of would be having Geno in your party and Boosting Mario, using a Yoshi-Ade (doubles attack), energizer (attack +50%), and get an "Attack Up!" bonus from killing an enemy. If all those items and things don't stack, well, then, use whatever does. I'll try everything.
I'm not sure which enemy has the lowest defense, but I'd try a Dry Bones, Goomba or an Enigma or maybe one of those blue, wobbly mushroom guys from the factory (SMRPG Legacy has them listed as Ameboids). Dry Bones would be great except for the fact that I don't think you can get an Att Up from killing one.
I currently have a level 22 Mario who's only had att bonuses, but no Super Suits or Yoshi-Ade's. I suppose I'll use some cheats to get those. I'll post a pic of whatever my best hit is so far once I get my SNES9X working as it should (no, I am not always holding down left!

).