the location guide says:
Corsola == Apophyll Beach: Event (Rain/Thunder storm required)
(Water, Rock)
But I can trade you one, if you don't want to wait for the right weather
Terra uses glitch field, check it here: http://pokemon-reborn.wikia.com/wiki/Glitch_Field
Dark type moves deals normal type dmg, so knock off won't be that helpful.
A special offensive Ice type will help you a lot in this match.
You could get an A-ninetales and/or an abomasnow.