Tamurlin

It's an item you pick up in the Grand Hall, in a pokeball in the top left corner.

I've been doing Fern's victory road puzzle according to a youtube guide, but now I'm stuck. So I turned trees into crystals three times and got Fern's Decuideye to break them with arrows, and from what I see from the video it should stop shooting arrows after the third time, which lets me move up the main path. But after breaking all three crystals the arrows haven't stopped, and I can't get through. I can't figure what I missed doing.

Should talk to Venam in Teila Resort, then when you enter the mountain a character should give you the Emotion Powder.

Did you meet with Venam in Teila resort before heading to the mountain?

Talk to Damien to get a ride back, and take the train on Route 9 to the tournament.

Go left after you enter Route 5. It's a little hard to see but you can move between the bottom of the first body of water and a row of trees.

Take the train to Route 9 from the station in GDC. Also, I'm currently unsure of how to proceed in

Where can I start this sidequest?