First get the Armoury key from the bridge outside the clock tower, or if it manages to fall down, in the floodgate outside the armoury. First you will fight a Vysage, so use the usual tactics to win, i.e cast Siren on it to stop it from using magic, then unload with your best attacks to finish it off. But after beating it, Gargantua appears, and will counter physical attacks with Counter Twist, and in addition, it can use Berserk on your characters to force them into only being able to attack physically, so use magic and GFs to beat him.
