That's outright false, Decimation has max priority it can't just be naturally outsped like that you need a higher priority move. Not only that, but Decimation isn't useless on any type, it's a completely custom move that's an OHKO like Fissure or Horn Drill or Sheer Cold. Also... how would Greninja get to level 100? There's a hard level cap now, the only way would be with 65 rare candies.
And though it might get you a Karma Point, the whole story doesn't change and she's not afraid of you. It kinda goes the same as if you lose really. If Decimation really didn't work then you might want to submit that as a bug report on the forum for the developers because that's not supposed to happen.