And while you're correct on a few levels, there are definitely a few that benefit from walling in on. I would say that the real "key," if there is one, is to choose a good order to do the missions in. There are a few missions that are VASTLY more difficult if you haven't got certain units yet. I won't tell you the order if you want to figure it out yourself, but for example, one of the earlier ones is quite difficult if you don't have siege tanks yet. One of the later ones is way harder if you don't have science vessels. etc. There are only 4-5 truly difficult missions, and, if you do missions in the right order, only perhaps two of those remain difficult.