Which one is the best for grind?

I’ve been playing online battles for a while now and don’t know if I should keep on playing ultimate battle and watch an ad every time (mostly I lose at the very end) idk which one is optimal for grinding?

I’ve been playing online battles for a while now and don’t know if I should keep on playing ultimate battle and watch an ad every time (mostly I lose at the very end) idk which one is optimal for grinding?