I am up to the bit where you have to fight tunnel armour.

Celes says to use runic.

But the option is greyed out.

Can someone help?

Equip her with a sword... she won't have Runic available if she doesn't have a sword.

I see...

Thanks

Runic kinda kicks, considering the later bosses: Kefka, that thunder demon that attacks Terra on WoR, any magic reliant monster, so Runic is mainly pretty good. It even helps out Celes's Cure.