At a minimum, every valuable card should be in a soft penny sleeve and then placed into a semi-rigid holder or a top loader
In this case, because your life total also can't change, that damage has any other effects that it may have aside from causing you to lose that much life (such as effects from lifelink or infect) and triggers and effects can see that damage was dealt even though your life total didn't change
This is a five-mana white enchantment that reads: If one or more creature tokens would be created under your control, that many 4/4 white Angel creature tokens with Flying and Vigilance are created instead. Basically, if you were going to make a 1/1 Goblin token, for example, you would instead make a 4/4 angel with Flying and Vigilance
But what will it mean for the overall health of the card game
Let them KO Xerneas after it did its job, then go off
Mewtwo Unleashed - Forme Y Psychic/Effect/DARK/Level 10 ATK 3000/DEF 3500 Cannot be Normal Summoned/Set