→ ring a bell
I never beat Alder so I can't agree or disagree with the Volcarona. However, I would have to agree with Bouffalant and Ghetsis' Hydreigon. I had to go out and get my Reuniclus to learn Focus Blast in order to defeat that blasted thing. Didn't help that I was underleveled. Other than that I don't really remember having any troubles with any other Pokemon. I guess Sigilyph earlier in-game was a pain and maybe overleveled for that point in time but I'm not sure.