The semi-hidden Odin call spell can be obtained any time after you receive a flying vehicle. Visit the auction house in Treno and look for the item "Dark Matter". (If it's not in the initial list of the day's items, just leave and re-enter.) Talk to the auctioneer and keep bidding until you win it. Equip it on Garnet to learn the Odin summon. As always, Odin attacks the monsters with an instant-death strike that either hits or misses.

   However, there's a way to power up Odin to deal damage as well. Take the middle staircase from the first room of Ipsen's Heritage. Follow the path until you reach a room with a diamond-shaped intersection. The chest on the left contains a Rod, another of the weak weapons find. Enter the door at the center of this area; you're now in a room with a huge sword (Odin's Zantetsuken?). There's a chest in here with Eiko's weakest flute as well as a pair of urns. First pick up the urn on the right and move it to the top cushion. Then move the urn on the left to the rightmost cushion, where the other urn used to be. Next, move the urn on the top cushion to the cushion on the left. Finally, move the urn on the right cushion to the top cushion. It will become charged with energy from the sword. Pick it up.

   The urn is now an item in your inventory. Garnet can equip it as an accessory to learn the Zantetsuken Damage status ability. When this ability is armed, Odin's Zantetsuken will deal non-elemental damage to any enemies it fails to kill outright.