If either target leaves the battlefield before the ability resolves, the ability taps the remaining target, but no damage is dealt. If both targets leave the battlefield, the ability doesn't resolve.

Magus of the Arena's controller always chooses their creature first.

If an effect changes the targets of this ability, the second target can be changed to any creature the chosen opponent controls. The choice of opponent can't be changed.

Tapped creatures can be targeted. Tapping them again will do nothing, but they'll still deal damage.

If an effect changes the targets of this ability, the second target can be changed to any creature the chosen opponent controls. The choice of opponent can't be changed.