Earth can be beaten using only orbital mines, fully upgraded blaster units, and towers. most of the units should be placed on top to the towers. the very top and bottom rows can be defended using ONLY orbital mines until the middle three rows have sufficient defenses built up. Use the first two planets to complete all necessary research. Putting Obama as the final boss was an unexpected touch.
After 19 boss plays with maxed out stats and equipment, I still cannot kill Atropos. 1/5 for being extremely frustrating. It got more tedious than fun at the end.