The final part was spotted from the sky by one of the Wardens... apparently it's lodged in the ice on the back of a wandering Alpha Avalugg.

Unfortunately, this means someone (or ideally many someones) need to either chase down and stop this unstoppable, immovable Pokemon in its tracks or redirect it someplace where it can be... er... okay, well, okay, just climbing on its back isn't going to work, and the ice up there is cartoonishly slippery, and the part itself is stuck...

This one is hard! Time to get creative.