Almost trivial. Modor is kind of badly coded and designed. Definitely use cold resistance, though.

Modor flies a while, uses a breath weapon in the air, and lands, where she can either claw or bite (if you are nearby) or use another breath weapon. The flying behavior works well enough. You can shoot her with arrows, particularly fire arrows, during this time. The breath weapon is nasty, but is pretty well telegraphed and relatively easy to avoid.

When Modor lands, she's barely able to move. Maintaining range is easy. So, there's no reason for you to even have to deal with the claw or bite attack. It isn't really a great idea to try to fight Modor in melee - both you and Modor will have trouble with vertical hitbox range, and stuff will unexpected miss, hit the ground, etc. (this is the badly coded bit). She also has a breath weapon which doesn't have a lot of range, and is still well telegraphed and easy to dodge.

So basically as long as you bring a ton of arrows, can dodge halfway competently, and don't try to mess around in melee range there's almost no difficulty at all. It was easily the lowest difficulty boss I've killed so far.