Is it possible to just have the amiibo fighting so they grind out levels super quick?
Obviously, it is possible in all smash bros since game cube. The Amiibo are literally another computer with a little more advanced artificial intelligence.
In Smash Bros Melee of the Game Cube you could configure four computers at level 9 so that they would fall and fight each other. What's more, it was the easiest way to unlock characters without going through classic mode.