I would make it a single target buff that fully blocks the next instance of damage you take. Naturally I would give it an appropriate cooldown to match. I realise that allowing you to outright ignore some mechanics would be somewhat broken, but I'd still look to make it an
absurdly powerful skill that's exclusive to White Mages.
As nice as flat damage reduction buffs are, you never really get the chance to see just how effective they've been. Having your tank outright ignore the next hit is something that is both noticable, powerful, and extremely satisfying when pulled off well.