Steal from the Guado Guardians so they can't use Auto-Potion. Then they're easy to kill. Dispel the Protect and Shell they cast when they die. If you don't know Dispel, use Ixion's Aerospark.

After that, use a combination of Power Break, Threaten, Blind and Sleep on Wendigo to keep him subdued while you whittle away his HP. When he raises his hands, stop attacking and switch to Lulu. Use Fire magic from now on.