As Boko posted, Morrigan's Anti-Magic Ward makes an ally immune to all magic for 10 seconds. But the spell has a cooldown of 30 seconds, so you have to make it count. I was thinking Lightning could use that time to cast. Not that she needs to straight-up win a casting battle against Terra -- just needs to knock her out of her flight path and into melee range. If she does that then I think she can unleash a DEVASTATING MELEE COMBO.
re: Graviga: According to the FF wiki at least, it doesn't work against enemies who are immune to Death. So Lightning is presumably okay. Dunno if Alucard has any anti-instant-death.
Worth noting that if Morrigan brings out Antimagic Ward, Terra can bust out Ultima. Considering Ultima pierces MDEF and punches through reflect in FF6, it at least is worth talking about and seeing which spell beats which. ---