Instructions
Lock onto Tiamat as she circles the cathedral's roof by holding down the focus button. Use the dash button to avoid the fireballs Tiamat will fire at you while she circles overhead. Locking onto Tiamat will allow you to use dash more effectively when avoiding her attacks then trying to manually adjust the camera as she moves.
Pickup an inert bomb from the platform and target Tiamat when she perches on the edge of the cathedral. Get as close to Tiamat as possible and then throw the bomb at the demon. The bomb will stick to the body of Tiamat if you successfully connect on your throw.
Target one of the torches on the rooftop with your Crossblade first, then target the bomb on Tiamat's body by clicking on the right control stick and using the targeting feature. Throw the Crossblade at the torch and the blade will bounce off the torch, catch on fire and head toward the bomb on the demon's chest. The burning Crossblade will ignite the bomb when it strikes, causing it to explode.
Advance toward Tiamat after she falls to the ground stunned. Begin attacking Tiamat with the sword, Chaoseater, as she lays prone on the ground. Dash away from the demon when she begins to stir.
Dodge Tiamat as she swoops down by moving to the opposite side of the rooftop, and dashing toward her as she begins her descent. Tiamat will swoop down at you three times before returning to shooting fireballs. Repeat the process of dodging fireballs, throwing a bomb, detonating it and attacking her when she falls stunned to the ground. Completing this process a second time will alter Tiamat's attack pattern for the rest of the fight.
Charge your Crossblade and continue throwing the weapon at Tiamat while dodging her fireball attacks until she falls to the ground stunned. Attack Tiamat with Chaoseater when she falls stunned to the ground. Repeat this attack pattern three times to end the battle with the demon.