We know that Motion Control may be the intended way to play ARMS but not the only method of play. ARMS will also incorporate single Joy-Con play along with the Pro Controller, in handheld mode both Joy-Cons & The Joy-Con Grip.
In most cases, A & B buttons are used for left-hand & right-hand punches. X is to jump and Y is to dash. Pressing A & B at the same time will activate the Grab and R is the Rush move.
Source: Nintendo Life