Stand your ground against the Bear at the first encounter.

This achievement can be unlocked during the Taiga level. After making your way through the earlier stages of the level you will eventually get to a gate in front of a church that you need to enter to progress. In this area you will encounter and be able to fight the Bear for the first time. It is possible to miss this as you can simply climb the rope ladder to reach the roof and avoid this fight, however doing so will of course void this achievement.

Fight off the Bear until it flees and the achievement will unlock.