If some, but not all, of the ability's targets become illegal, you can't change the division of damage. Damage that would've been dealt to illegal targets simply isn't dealt.
Polukranos, World Eater
Monstrous isn't an ability that a creature has. It's just something true about that creature. If the creature stops being a creature or loses its abilities, it will continue to be monstrous.
You choose the division of damage as you put the ability on the stack, not as it resolves. Each target must be assigned at least 1 damage. In multiplayer games, you may choose creatures controlled by different opponents.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Johann Bodin
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
