Skarmory's normal set is using Stealth Rock, Spikes, Whirlwind and Roost.
Their EVs are 252 HP / 252 Def / 4 SDef with Bold Nature (+Def, -Atk), helding a Rocky Helmet and Sturdy Ability.
I'm going to test with Salamence as you said and going to give you some good counters..
Physically Defensive set
252- Atk Salamence Fire Fang vs. 252 HP / 252+ Def Skarmory: 76-90 (22.7 - 26.9%) -- possible 5HKO after Leftovers recovery. - 252 Defense EVs + 252 HP EVs
252- Atk Salamence Fire Fang vs. 0 HP / 252+ Def Skarmory: 76-90 (28 - 33.2%) -- 88.9% chance to 4HKO after Leftovers recovery. - 252 Defense EVs + 0HP EVs
Specially Defensive set
252 SpA Salamence Fire Blast vs. 252 HP / 252 SpD Skarmory: 212-250 (63.4 - 74.8%) -- guaranteed 2HKO after Leftovers recovery. - 252 Sp. Defense EVs + 252 HP EVs
252 SpA Salamence Fire Blast vs. 0 HP / 252 SpD Skarmory: 212-250 (78.2 - 92.2%) -- guaranteed 2HKO after Leftovers recovery. - 252 Sp. Defense EVs + 0HP EVs
Skarmory has Bold Nature in the 4 tests and survived all hits.
Remember that Skarmory is outspeed by many threats like Salamence, Talonflame.
Good counters for a 252 HP + 252 Def or 252 Sp.Def
Physically Defensive - +2 252 Atk Life Orb Talonflame Flare Blitz vs. 252 HP / 252+ Def Skarmory: 421-499 (126 - 149.4%) -- guaranteed OHKO. - 252 HP + 252 Def
Specially Defensive - 252 SpA Mega Charizard Y Fire Blast vs. 252 HP / 252 SpD Skarmory in Sun: 620-732 (185.6 - 219.1%) -- guaranteed OHKO. - 252 HP + 252 Sp.Def
So to your 2 questions, you should bring a Salamence with Fire Blast (accuary isn't so good) and the Skarmory's normal role is physically defensive. Salamence's Fire Blast is 2HKO to Skarmory and in the sun is OHKO. Remember that the Salamence has 252 Sp.Atk EVs.
The counters are good choice to! Talonflame with Swords Dance and Life Orb is a monster, however with the recoil damage, it only will OHKO a maximum of 2-4 Pokémon with max health.
Hope this helped.