I've never been able to figure out how to use Celes' Runic skill to any effect in battle. Is it for a secret boss? What does it do?
Celes' Runic ability will absorb the first spell cast by anyone during the time that it is active, and use that to replenish her MP. If no one casts a spell, then Celes effectively wastes her turn.
The advantage to this skill is that you can avoid extremely damaging magic attacks with it. For instance, if you are worried that a Brachiosaur is going to use Ultima on you, using Runic will protect your party from taking damage, while giving Celes a MP boost.
There are certain restrictions, however. Only certain weapons work with this skill. For instance, all swords, spears, daggers and katanas are compatible with it. Although it works with almost all magic spells (ie, those that your characters can learn via Espers), not all enemy skills are absorbed with it, although most are.
For a list of all the special cases and other info regarding this skill, consult the relevant Final Fantasy Wikia article.