Mission: Central Boulders
Medal: Triple Kill
Step 1: Walk to the left and shoot the LEFT boulder (so it will fall and block the lava).
Result: After Shooting the LEFT boulder, it's the Evil Robot's Turn they will shift and cluster into a threesome. Usually when they shoot they will miss your players.
Step 2: Shoot the RIGHT Boulder, and it will fall and tumble on the Cluster of Three Evil Robots! Tadaaaa!