You will be awarded this achievement for completing the mission "Cliffhanger" on any of the difficulty levels. This is the third mission from Act 1. You start off by climbing up the ice using the right triggers. You will need to hold them will going up otherwise you will fall to your death. Listen closely to all orders given and you will easily enter the base. Once in the base you will need to plant some C4 then meet back up. Then you will need to fight your way to the snow mobiles and race to the LZ.