Recommended Level: 16+
The second Titan you're recommended to tackle on the Path of Legends Treasure Hunt, the Sky Titan will take you into the rocky western lands of Paldea. If you opted to pursue the VIctory Road Treasure Hunt on your first foray out of Mesagoza, going after the Open Sky Titan first isn't a terrble idea, as it's only one area over - and after battling the Gym Leder of Cortondo, you're probably ready for the challenge.
To find the Titan you need to make your way through West Province (Area One), which is found to the northwest of South Province (Area Two). On the western edge of Cortondo you'll find a path leading into the area. Once that's done, follow the main path along the winding cliffs of the region until they bring you to a Pokémon Center. Follow the path a bit further west, until you see windmills, then go north.
The path will split ahead, and at this point you need to leave the main road behind. Look to the northwest, up a river, and you'll see... something... way in the distance, dropping rocks down a steep hill. You'll get a message from Arven at this point, giving you some details regarding the Titan. Needless to say, the shape in the sky is the Titan.
Jump onto your mount and ride up the hill, avoiding boulders as best you can. Once you get pasr the drop point you'll run into your opponent. Battle!
Bombirdier, the Open Sky Titan
A Flying- and Dark-type Pokémon, Bombirdier is a fairly strong opponent early in the game. It primarily uses Pluck and Wing attack to do Flying-type damage, making it a threat for anything vulnerable to Flying-type attacks. A Rock- or Electric-type Pokémon can clip Bombirdier's wings without a ton of trouble, so long as you watch their health.
Do enough damage and Bombirdier will slam open the cliff behind it, revealing an Herba Mystica. It'll eat it, and the battle will begin again with Arven helping out. Arven will throw out a Nacli that knows Rock Throw, and despite its size the Nacli can wipe out the Titan more or less on its own. Either pitch in or sit back and watch the little creature do its thing.
Defeating the Open Sky Titan will earn you a Titan Badge, and Arven the Bitter Herba Mystica. He'll convert it into sandwiches for everyone, including your mount Pokémon. If this is your second Titan defeat your mount will now be able to move across water without sinking, which makes navigation in certain sections much, much easier.