When Ethersworn Shieldmage’s triggered ability resolves, it doesn’t lock in which permanents will be affected. For the rest of the turn, any time damage would be dealt to a permanent, check whether it’s an artifact creature at that time. If it is, that damage is prevented.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.