The battlefield, a grassy plain. The time, mid-day.
Mario & Luigi have both each obtained a Fire Flower and are sporting their white overalls and hats. Ace is capable of doing everything we've seen of him thus far in the manga.
Ace deffinately has more firepower at his disposal, which is why I've made this two against one. I think as a team, Mario and Luigi are capable of taking Ace out, especially is Ace decides to take one of his oddly timed naps.