Recommended Level: 44+
Secreted somewhere in the Asado Desert, on the western shores of Paldea, the Quaking Earth Titan is likely the fourth Titan you'll pursue on the Path of Legends Treasure Hunt. The area surrounding the Titan is filled with tough desert-dwellers, many of them on the dry side, so bringing along a Grass- or Water-type Pokémon is recommended.
The Asado Desert is sandwiched between Porto Marinada, in the north, and Cascarrafa, in the east. You'll likely first reach this area by traveling through the southwestern sections of Paldea, notably South Province (Area Two) and West Province (Area One). As long as you don't speed through these two areas you shouldn't have any trouble handling the denizens of the Asado Desert. You'll find the larger, more notable entrance to the desert in Cascarrafa.
Trek west out of Cascarrafa and straight through the desert. After a few minutes of walking west you'll get a call from Arven, providing the few details he has about the Quaking Earth Titan. Carry on west until you're south of the exit to West Province (Area Two) and, more or less Porto Marinada. Over the next hill to the west is a huge rock formation, and near it you should see a huge Pokémon slowly roaming the desert. Run right up to it to engage the Titan.
How this fight goes depends on your version of the game, and the Donphan analogue that you battle varies greatly between Pokémon Scarlet and Pokémon Violet.
Great Tusk, the Quaking Earth Titan (Pokémon Scarlet) / Iron Treads, The Quaking Earth Titan (Pokémon Violet)
That's a hefty boy. Both Great Tusk and Iron Treads use physical moves to smack your Pokémon around, including Knock Off, Rapid Spin, and Brick Break (Great Tusk) or Iron Head (Iron Treads). Its attacks are extremely powerful, and even a weaker move like Rapid Spin can knock out your Pokémon in a single strike. Great Tusk is a Ground- and Fighting-type Pokémon, making Water-, Grass-, Psychic-, Ice-, Fairy, and Flying-type moves a good bet for surviving the fight. Iron Treads is Ground- and Steel-type, so go for Fighting-, Ground-, Fire-, or Water-type moves instead. Deploy Pokémon with high defenses, if possible, and use Status ailments like poison and paralysis (though mind that poison won't do anything to Iron Treads).
The Titan will vanish once it's defeated, and you'll find it at a huge rock formation to the southwest of where you fought it. It will smash open the rock, eat the Herba Mystica inside, and reengage you in combat. This time you'll have Arven helping out, and he'll deploy a Scovillain to aid you. This time the Titan can use Stomping Tantrum to inflict Ground-type damage and Scary Face to slow your speed. If you don't have the hitting power to take Great the Titan down your best bet is to use your team to stall while Scovillain does most of the damage. The Titan won't target it very often.
Defeat the Quaking Earth Titan, whatever its form, and you'll earn another Titan's Badge and the Sour Herba Mystica. Arven will concoct his usual sandwich, and from this point on your mount will be able to glide while in the air by pressing B.