All levels are playable from the offset, so if you want this achievement right away, go ahead and pick any level apart from 'Ghost in the Machine' or 'Divided Attention.' An excellent playlist of gold medals on all maps and modes can be found here. If you decide to try going alone, there are many different ways to complete each and every level in this game to a gold medal standard. Remember to always ensure that the aliens are taking the longest possible routes so you have more time to kill them. If you make a mistake, you can hit to go to your last checkpoint; and hit it multiple times to go back further.