Nintendo have stressed that motion controls are the proper way to play ARMS, but single Joy-Con usage is also an option. This is not the preferred method and will definitely hinder your fighting ability, but is still there for those spontaneous multiplayer sessions with friends.
EDGE magazine’s latest issue notes that:
‘Punches can be thrown using the triggers, or two of the face buttons – jump and dash are mapped on to the other two. You click the left stick to guard. Everything you need is here, and it works – but something’s missing. Since the left stick controls the angle of your punches, you can’t move in one direction and punch in the other. You can’t strike at a different angle with your other hand until the first punch is fully extended, either.’