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Phantom Wurm enters the battlefield with four +1/+1 counters on it.
If damage would be dealt to Phantom Wurm, prevent that damage. Remove a +1/+1 counter from Phantom Wurm.
If Phantom Wurm is dealt damage by multiple sources at once (most likely because it was blocked by multiple creatures in combat), all of that damage is prevented and you remove only one counter from it.
If an effect raises Phantom Wurm’s toughness so that it remains on the battlefield without any +1/+1 counters on it, Phantom Wurm’s ability prevents all damage that would be dealt to it even though you can’t remove a +1/+1 counter from it.
If damage that can’t be prevented is be dealt to Phantom Wurm, you still remove a counter even though the damage is dealt.
