This achievement is earned in the Abandoned Mill level, which is pretty early in the game (right after the Thieves' Forest level).
At the start of the level, you will mount a deer that you will ride through a short side scrolling area as you are chased by a giant troll. You must make it through the level without hitting any obstacles (logs, walls, etc) in order to earn the achievement,
If you're playing solo, there will be three other deer running along with you that are controlled by the game. You can use these deer to determine the location of the doorways as they will align themselves when one is coming.
You can exit to map and restart the level until you get through without any mistakes.