x2Shuriken Flurry
When you play this Pokémon from your hand to evolve 1 of your Pokémon during your turn, you may put 3 damage counters on 1 of your opponent's Pokémon.
Haze SlashYou may shuffle this Pokémon and all cards attached to it into your deck.
Shadowy Hunter-GXThis attack does 130 damage to 1 of your opponent's Benched Pokémon. (Don't apply Weakness and Resistance for Benched Pokémon.) (You can't use more than 1 GX attack in a game.)
Pokémon-GX rule: When your Pokémon-GX is Knocked Out, your opponent takes 2 Prize cards.
If I use Greninja GX's "Haze Slash" attack to shuffle it back into my deck and also Knock Out the Defending Pokemon, who promotes a new Active Pokemon first?
In this case you promote first. Greninja GX is shuffled back in as an effect of the attack before KO's are checked, so it must be replaced before either player checks for KO's.