Once you reach the freeway on Route 4, you will find a road going west. This leads to a small house with two workers on it, and a northward road that goes to Desert Resort.
Like Route 4, there is a constant sandstorm in Desert Resort. It's a large area, and there are a bunch of trainers all around. There are also two entrances to Relic Castle.
Near the entrance of Desert Resort is a Doctor. He is a trainer, and he has a Level 23 Solosis. This is a pure Psychic type pokémon, so use Dark or Bug moves.
Once you defeat the doctor, he will heal up your pokémon for free whenever you talk to him.
Pokémon that you can catch in Desert Resort
The wild pokémon in this area are Sandile, Darumaka, Maractus, Scraggy, Dwebble, and Sigilyph. The Sigilyph is difficult to catch because it has the Whirlwind move, which immediately ends the battle. You can either try to throw a powerful pokéball at it, or you can try to wear down its HP and inflict Sleep on it quickly, in the hopes that it won't use Whirlwind before you get a chance to throw a pokéball. You always encounter it at level 20 here, so using a Nest Ball on it will be the same as using an Ultra Ball. However, if you have Dusk Balls and try to catch Sigilyph at night, you have an even better chance.
Be sure to have your Dowsing Machine on while you go through the desert, because there are plenty of hidden treasures around. Keep an eye out for non-hidden treasures as well.
Darmanitans with Zen Mode hidden ability
One of the entrances to Relic Castle has a bunch of Darmanitan statues around it. As soon as you get a RageCandyBar, which you can get in Icirrus City in the Winter, you can return to these statues and check on one to feed it the RageCandyBar. It will awaken as a Darmanitan that has the hidden Zen Mode ability.
Zen Mode will make the Darmanitan change form if its HP is less than 1/2 its maximum. While in Zen Mode, the Darmanitan's type is Psychic/Fire.
These particular Darmanitans know Flare Blitz, Thrash, Belly Drum, and Hammer Arm (Fire, Fighting, and Normal attacks). Belly Drum cuts its HP by 1/2, and keep in mind that the sandstorm will gradually wear away at the pokémon's HP. Flare Blitz also inflicts recoil damage on Darmanitan, so don't let Darmanitan's HP get too low before throwing Pokéballs at it. You should try to inflict a status condition on the Darmanitan and lower its HP, then throw a Dusk Ball at it if it's night, or an Ultra Ball otherwise. See other pokéballs to use on Darmanitan.
If you can't get the RageCandyBar in Icirrus City yet, the next opportunity that you will have to get the RageCandyBar is after you receive the National Pokédex. After you get it, Walk up to the Relic Castle entrance near the Darmanitans, and Professor Juniper will arrive and give you a RageCandyBar.
After getting the National Pokédex, you will also be able to get RageCandyBars on the Royal Unova in Castelia City. The Royal Unova is a ship that departs from one of Castelia City's piers. You have to go on Saturday and defeat all of the trainers on board in order to receive the RageCandyBar. The prize is different on other days, so be sure to go on Saturdays to get the RageCandyBar.
Getting to Relic Castle
Around the northeast and northwest areas of the desert are the entrances to Relic Castle. You can fight a couple of Psychic Trainers here, but you won't be able to go much farther. A worker stops you near the stairs leading down, saying that there is too much sand for you to proceed. To get past the sinkholes without falling in, walk (don't run) along the edge, avoiding the center. However, if you do fall in, you will find a revive and will be able to take the stairs back up to the entrance.