Dragon Slayer is an iconic quest, and in BattleScape we only have the boss fight. For iron man and hardcore game modes, this shop is the only place to buy mystic armor, the four fremennik helmets, green dragonhide and proselyte armor.
Starting off, you are strongly advised to bring an anti-dragon shield; without it, you may be hit for over 20 hitpoints. If you do not know where to get an anti-dragon shield, it is sold by Thessalia at the shops for 95gp.
The quest:
Start off by heading south of the home bank to the wall of quest NPCs.
You will be talking to the Guildmaster.
In his dialogue options, choose "miniquest" to begin.
You'll end up inside of a volcano, just outside of the city of Tzhaar (not seen).
Following the path while ignoring some level-22 skeletons you'll find Elvarg (level-101) behind some stalagmites.
Climb over the rock wall and get ready for your fight! It's very easy, I did it on this account with 60 attack, 63 strength, 57 hp, 1 defense, 1 prayer and no food (though you see I brought some! I wasn't even potted up!)
Congratulations! You've completed the Dragon Slayer quest!
The shop:
Rune full helm: 21,200gp
Rune platebody: 39,000gp
Rune kiteshield: 32,700gp
Green d'hide body: 4,680gp
Green d'hide chaps: 2,340gp
Green d'hide vamb: 1,500gp
Mystic hat: 11,800gp
Mystic robe top: 72,000gp
Mystic robe bottom: 48,000gp
Mystic gloves: 6,000gp
Mystic boots: 6,000gp
Mystic hat (dark): 10,300gp
Mystic robe top (dark): 72,000gp
Mystic robe bottom (dark): 48,300gp
Mystic boots (dark): 41,200gp
Mystic hat (light): 9,000gp
Mystic robe top (light): 72,000gp
Mystic robe bottom (light): 48,000gp
Mystic boots (light): 6,720gp
Amulet of power: 2,480gp
Proselyte sallet: 4,800gp
Proselyte hauberk: 8,690gp
Proselyte cuisse: 8,470gp
Splitbark helm: 6,000gp
Splitbark body: 27,000gp
Splitbark legs: 24,000gp
Splitbark gauntlets: 3,000gp
Splitbark boots: 3,220gp
Berserker helm: 53,000gp
Warrior helm: 54,300gp
Archer helm: 37,400gp
Farseer helm: 36,000gp