Another really easy Atlas to hijack is the Mech that is dropped near the end of the "From Ashes" DLC. When it gets dropped near the life pod, I just hid in the building until it came close, pulled out my shotgun and let loose on the glass and broke the glass. Once the glass was broken, I took out my assault rifle, killed the driver and hijacked it. Just got the achievement! :) Just an alternate way.