Go to Exire. Go behind elder's house (it looks same as the other houses but it's above few steps). Walk forward a narrow path until you arrive to a grave-looking stone. Equip Ruby, Aquamarine, Opal and Garnet to your first four characters and then examine the stone.

Maxwell:
HP: 60 000 TP: 800
EXP: 10 800 Gald: 8250
Strengths: none
Weaknesses: none
Techs: Dreaded Wave, Raging Mist, Gravity Well, Meteor Storm

This is a king of all Summon Spirits and you must have a Derris Emblem to battle him. When he uses Meteor Storm, use guard tech three times to guard it. You can evade other techs like normally. Maxwell isn't very hard, so this battle shouldn't be a problem. Recommended level for this is 60-65.