Strategy:
Throw Sleep Seed at Dusknoir. There's a reason why he's respected. Because he's deadly.
If you've taught one of your team Dig, it'll really help against the Sableyes. Every now and then, check your teammate's HP. If low, give them Oran Berries to help them stay quirky. By the time Dusknoir wakes up, you should have taken down at least 3 Sableyes.. They're not too hard, actually.
Use Dark moves like Bite and Crunch to keep Dusknoir at bay. If you have a Grass type, Absorb or Mega Drain become really effective when you lose HP. If not, but you have a starter for a partner, rely on the Overgrow/Blaze/Torrent abilities if you don't want to waste Orans.
The key is to be in good shape. A team of level 30+ is recommended, and have several Reviver Seeds in stock. I didn't have any, so I won by luck...and speed.