Have all enemy skills inflict an invisible state.
Have a common event run during each action in combat, on every encounter that checks whether the actor has the shield state on, and if so, checks if they're inflicted with the invisible state from enemy attacks, and if yes, count the shield durability variable down by 1 (set up the shield durability in a common event linked to the shield skill (if it's meant to take 5 hits, set the variable to 5). Next, have it check if the variable is at 0, and if so, remove the shield state. At the end of the event, always remove the invisible state, so it can check for the next attack.
It'll be a pain to set up initially if you have to backtrack a lot of encounters, but I'd always recommend having common events run during turns and actions on all battle encounters to deal with these things.