Odin is a very tough foe, and fortunately you don't actually have to defeat him. You will charge his gestalt gauge with healing Abilities, but simply surviving isn't enough to win. You need to take advantage of lulls in his attack (especially when he uses Ullr's Shield), and keep up a chain while healing as much as possible. His attacks are devastating, so defensive items such as Silver Bangles, Spark Rings, or Black Belts can be a huge help. If you fail, play with your paradigm deck and equipment and try again. This is more of a puzzle than a fight, so keep at it until you're victorious.