Originally Posted by Fat Cefiro
So you empty the room, save, then you RNG when in the groto?
And that generates what you want?
That's right. Save immediately after emptying it (after either picking up the item or battling the Pokémon), then hit your seed and walk 256 steps. Leave the grotto, come back in, and you should see the Pokémon you want. If you don't see it, then either you hit the wrong Timer0 or you made a mistake somewhere.