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This creature can't be destroyed by lethal damage unless lethal damage dealt by a single source is marked on it.
The check for nonlethal damage is done on a per-source basis. So each source’s damage is checked independently and it is not possible to team up sources (such as creatures) in order to kill it.
If its toughness falls to zero or less and it is undamaged, it will still be put into the graveyard.