iirc you need a attack with 3 or more of damage, i think the first time i do it was with the lance. EDIT: "If you're trying to find permanent possession, it's on the far right side of Route A11, somewhere. You'll need a strong enough attack."
Figured it out! Apparently I needed some talisman first.. That was indicated prior. Thanks for the help anyways!
And with that, I have unlocked everything currently in the gallery!!
