Deity of Scars's ability causes it to have more toughness when you activate the ability (as a result of losing a -1/-1 counter) and then puts a regeneration shield on it when the ability resolves. If you activate the ability in advance of Deity of Scars being dealt 5 damage, for example, Deity of Scars will be 6/6 before the damage is dealt to it. It won't use the regeneration shield unless it's dealt more damage that turn.