my guess is that there was either some abnormal EV investment going on, or Gardevoir had a little over 55.2% HP, which would be just enough to survive a minimum damage roll Stone Edge if the Regirock had little or no Attack investment. Unfortunately, we can't see the actual HP number so there's no way to tell on the latter
That was a neat battle. You have some cool Pokémon.
How did Gardevoir at half health not faint from a Stone Edge from Regirock?
my guess is that there was either some abnormal EV investment going on, or Gardevoir had a little over 55.2% HP, which would be just enough to survive a minimum damage roll Stone Edge if the Regirock had little or no Attack investment. Unfortunately, we can't see the actual HP number so there's no way to tell on the latter