Depending on how much you’ve prepared for this battle, it can be quite difficult or pretty easy. As with the previous battle against Belgemine’s Ifrit, you do not need to win and the rewards for winning aren’t exactly all that great. You will actually be able to use both Ifrit and Valefor here, so if one goes down, you can still summon the other one. The only two things you will see for the first half of the battle will be a normal physical attack and Aerospark, which is also physical damage. At 50% health, Ixion will cast Haste on itself, meaning it will get a lot more turns now.
Ifrit is the better option to start this battle, as you can use Fire on himself to replenish his HP. If you happened to come into the battle with Yuna’s Overdrive ready, then a Hellfire from Ifrit should be able to take out around half of Ixion’s HP (it has 6,000). Depending on the stats of Yuna and subsequently, Ifrit, you should get another opportunity to use Hellfire, meaning it’s possible to two-shot Ixion here. Note that using an Aeon’s Overdrive will likely give Ixion a lot of turns in a row, so hopefully you’ve been building up Yuna on the Sphere Grid.