Use the bucket lift on the CASPIAN level to reach the Oasis.

This achievement is unlocked during the Caspian level when you are helping Damir to infiltrate the bandit camp to steal a tank of water. After following Damir through the early stages of the level, you will be given free reign to proceed how you want. You will be able to see the bucket lift in the first area with human enemies; it is prominent in the level. You simply need to approach it and press 'X' to ride it to the top.

Note that completing this level without hurting any slaves is necessary to unlock "Damir" and using this lift will help you to circumvent a good deal of enemies and help you to avoid hurting any slaves in combat. See its description for more details.