Rule 704.8

If a state-based action results in a permanent leaving the battlefield at the same time other state-based actions were performed, that permanent’s last known information is derived from the game state before any of those state-based actions were performed. Example: You control Young Wolf, a 1/1 creature with undying, and it has a +1/+1 counter on it. A spell puts three -1/-1 counters on Young Wolf. Before state-based actions are performed, Young Wolf has one +1/+1 counter and three -1/-1 counters on it. After state-based actions are performed, Young Wolf is in the graveyard. When it was last on the battlefield, it had a +1/+1 counter on it, so undying will not trigger.