Give it a State that lasts 1 turn (or however long), has a Physical Damage Rate of 0%, Magical Damage Rate of 0%, State Rate of 0% for all negative states, and have the skill apply that state to all allies.
Then, also create a State that Seals this skill. Have the only Remove Timing on this state be "Remove at End of Battle". Either have a damage formula deal 0 damage and apply this state to the user - the formula you'd use is a.add_state(n); 0 where n is the State ID of the state that seals the skill... or, if you know which character is using the skill, simply create a Common Event that applies this State to the skill user, and add that Common Event to the skill.