Giggling Skitterspike
Indestructible Whenever this creature attacks, blocks, or becomes the target of a spell, it deals damage equal to its power to each opponent. {5}: Monstrosity 5. (If this creature isn't monstrous, put five +1/+1 counters on it and it becomes monstrous.)
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Official rulings (3) · Scryfall
If Giggling Skitterspike is no longer on the battlefield when its second ability resolves, use its power as it last existed on the battlefield to determine how much damage it deals.
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.
Once a creature becomes monstrous, it can't become monstrous again. If the creature is already monstrous when the monstrosity ability resolves, nothing happens.