Extreme Adventure Zone
Reward: Refined Tortoise Gem.
Extreme Adventure Zone is Original to Serenity CO Online. Filled with Basilisks on steroids and other magic monsters, it is intended as a place for Archers and Taoists to be able to level past 120.
The NPC GeneralSergeant in the NightDevil/Robot map just past MetZone will let you in for free.
While your leveling, you might as well gather up 5 HeavenRunes and hand them into GeneralSergeant for a Refined Tortoise Gem.
Rates on Boss enemies are slightly higher.