This achievement is for Chapter 1, Level 1 in a level called Trespassers. (Not the prologue chapter). This can be done in solo or co-op but co-op is recommended because the other player can push over trees to clear the way for the player getting the achievement. Basically, the only thing that can push trees over is by walking into them. Luckily your gunfire and other weapons don't knock them over. Have your teammate lead the way pushing over trees to make an open path for you. Also, its advisable to do this achievement on easy so that you don't have to worry about dodging enemy fire, you can focus on dodging the trees instead. It's really an easy achievement though in co-op.