Here's a good set for Shelgon...
Role: Dragon Dance Sweeper
Ability: Rock Head
EVs: 252 HP / 20 Atk / 236 Sp. Def
Impish Nature (-Sp. Atk, + Def)
- Dragon Dance
- Outrage / Dragon Claw
- Rock Slide / Brick Break / Zen Headbutt
To be honest, this is the only viable set you're going to see on Shelgon. With Eviolite boosting its Defenses to a base of 150 / 75, Shelgon can be set up D-Dance reliably. The only problem is that its lack of recovery makes it easy to wear down.
Dragon Dance is obvious on this set. It boosts the Attack and Speed by quite a bit. After a few D-Dances, Shelgon can become a dangerous sweeper.
Shelgon needs to have Outrage for the maximum damage output. Unfortunately, it locks it into a single move, making it easy to counter. Dragon Claw is a safer alternative, but it doesn't hit as hard as Outrage.
Double-Edge gets good damage and doesn't have to worry about recoil (because of Rock Head).
Brick Break deals with Rock-types / Steel-types that can wall this set. Rock Slide gives random coverage, and so does Zen Headbutt.
The only other set you're going on Shelgon is a Wish pass set (with EVs: 252 HP / 4 Def / 252 Sp. Def and Calm Nature). This set has Wish / Protect / Roar or Toxic / Dragon Claw.
Its Base stats with the Eviolite boost would look something like 65 HP / 95 Atk / 150 Def / 60 Sp. Atk / 75 Sp. Def / 50 Speed.
Salamence obviously works much better for a D-Dance sweeper because of its base stats 95 HP / 135 Atk / 80 Def / 110 Sp. Atk / 80 Sp. Def / 100 Speed. Because of its well-rounded stats, it is able to pull off a Mixed Sweeper set and a D-Dance set.
OR / AS gave a significant improvement on Salamence by granting it a mega stone. Now with the base stats 95 HP / 145 Atk / 130 Def / 120 Sp. Atk / 90 Sp. Def / 120 Speed, this allows it to abuse both a Bulky D-Dance set and a Offensive D-Dance set.
Dragonair has base stats (with Eviolite boost) 61 HP / 84 Atk / 97.5 Def / 70 Sp. Atk / 105 Sp. Def / 70 Speed. Although that doesn't look like much, it is chosen primarily because of its two good abilities: Marvel Scale and Shed Skin. This makes it a good Status absorber and a RestTalk D-Dance sweeper and probably is why it is used more than Shelgon. In addition, Shelgon can easily get worn down from Status Conditions, while Dragonair has Shed Skin to protect it.
In conclusion, Shelgon is NOT viable for competitive play. The only time you're going to use is on Showdown; tier PU. You should definitely use Mega Salamence (or Salamence if you're playing OU / UU) instead of Shelgon because of the raw power it offers and the fact that it has similar bulk to Eviolite Shelgon.