Unlockable within FOREST GROVE
There will be plenty, plenty of opportunities to defeat 15 enemies in a row without taking damage.
Just use the bow and keep your distance (I shot all the wolves that came at me). To make it easier, there is even a kill streak counter that shows up on the screen. The only way the counter resets for this is if you get hit by an enemy. If you happen to fall off the map or into some water, the counter is not reset.
Watch out for the tree guys. They shoot projectiles at you, and right before they die, they shoot projectiles out from all around them. Also watch out for poison sacks. You will encounter them the first time you enter the portal and go into the dark forest. Keep back and shoot them with your bow.