Before you finish mission 2 as MacDonald, make sure you are wearing the goon uniform. Slide down the zipline to end the mission. When the mission 3 starts, you should be wearing the goon uniform. This will enable you to easily race to the hut in front of you to answer the phone, and prevent the mountain forces from hearing about you in advance. You can now easily kill the first few guards on the mountain in disguise. However, make sure no other guard sees you killing them. The disguise works until you have to past the two guards near the spotlight.