Cheats used to raise both bros. attack to 999, though that is perfectly attainable normally, with enough StarBeans cafe drinks. No other modifications were used.
I figure this attack has to be the most damaging of any attack in the game if done right, as doing the Advanced command adds up all of the single fireballs' damage (including the extra from the Mush Badge AA), and it can do double damage on certain enemies, of which Fighter Flies had the lowest DEF. Obviously, getting eighteen fireballs is impossible to do by button-mashing normally, though, so this is only the best way in theory.
Both bros. were equipped with Mush Badges AA (with a full inventory of Mushrooms of all kinds), Great Force and Red Peppers nearly tripled the resulting damage.
BTW, fun fact: Damage dealt is a 16-bit signed integer, ergo, 32,767 damage is the most it will display; any more will cause it to roll over to negative, and thusly do 1 damage.
![](https://i.ytimg.com/vi/RrruaYDriOs/mqdefault.jpg)