You get this for blowing up the dam. The trick here is once given the mission, follow your arrows (with the compass turned on) and you'll start jumping from platform to platform. continue this route until you reach the back wall and walk through a tunnel that leads to more platforms. once back on solid ground again you must continuously wall jump using a to start and x to keep bouncing upwards. at the top follow the final platforms and you'll see the explosives already strapped to a giant rock. throw a ball and the rock will crush the dam giving you the achievement