for this achievement you need to find a skull on any level. I think the easiest one to get is on the first level: A new enemy. The skull can be found by taking over 4 of the enemy power station the first one is found directly south of the first base you make. Once you capture the first power station its easy to find the rest as they appear on your mini-map as red triangles. Once you capture all of the power station the skull and the achievement is yours.
If you would like a video guide to show you how to get this skull I will leave mine below!