Recommended Level: 13+
Your (recommended) first step on the Path of Legends Treasure Hunt is The Search for the Stony Cliff Titan. An enormous Pokémon of relatively unknown description lives deep within the craggy cliffs to the east of Mesagoza, and Arven, ever pushy, needs you to flush it out for him. Sounds like a plan.
This quest will require you to travel to South Province (Area Three). Located through Mesagoza's eastern gate, South Province (Area Three) is a twisting maze of narrow tunnels through heaps and heaps of red-orange rock. For your purposes, however, you can skip most of the confusing side trails. Simply follow the path out of Mesagoza until it takes you to a Pokémon Center to the east of the city.
Once you reach the Pokémon Center you'll get a call from Arven who will share the few details he knows about the Titan. Head north once you're done speaking to Arven and you'll find a ridge that leads down into a valley. If you go east from here you'll leave the area and enter South Province (Area Five), so head towards the valley... but don't go all the way down.
Continue to follow the path from the Pokémon Center rather than going off the beaten path. This path meanders east, and will eventually guide you to the city of Artazon. Continue to follow it until you're about halfway between the Pokémon Center and Artazon, south of the marker for this quest. Look north and you'll see an absolutely enormous Klawf clinging to a cliff wall. This, unsurprisingly, is the Titan.
Approach the Klawf and it will scuttle up the wall. Follow the path on your right - maybe stopping to grab the Rare Bone as you go up the hill - and you'll find the Klawf clinging to another wall. It won't flee this time, though, and you'll find yourself facing off against the behemoth crab.
Klawf, the Stony Cliff Titan
Despite its intimidating size, this Titan isn't too difficult. Klawf uses a combination of Vice Grip, Rock Tomb, and Rock Smash to deal damage to your team, and will use Block to keep your lead Pokémon from escaping the battle. It's nevertheless just a Rock-type, and Water-, Grass-, Fighting-, and Ground-type moves will all do good damage to the Klawf. Inflict enough damage and its Anger Shell Ability will kick in, raising its attack stats but lowering its defenses, which will arguably make the fight easier since the Klawf is slow.
Do enough damage and the Klawf will flee down its rocky perch. Chase it and the Klawf will slam through some rocks, grab the Herba Mystica Arven wants, and power itself up. You'll now fight side-by-side with Arven to take the Titan down. Arven will toss out a Shellder to provide Water Gun support to your side, and between the two of you defeating the Klawf shouldn't take all that long.
(Ironically enough, catching and using another Klawf isn't a terrible way to wage both of these battle. The Titan Klawf's attacks won't do that much damage, and your own Klawf can use Rock Smash to whittle down its health pretty quickly. The Skidoos that frequent this area are also a good choice.)
Once the Titan is out of the way you'll find a cave, and inside Arven will discover the Sweet Herba Mystica. He'll give you the Titan Badge as thanks, and make you a nice sandwich that will go to your mount Pokémon. This will give it the ability to dash by pressing down the left control stick, allowing you to make more rapid progress through Paldea. Nice.