You need to get perfect on 100 different waves. To get perfect, you must complete the entire level without taking any damage at all. This will come with time. The first 20 levels of each revolution on each planet are quite easy to get perfect on as there are not that many enemies and they are generally week. If you want to get this out of the way, just replay any waves early in the revolutions on all planets (first 20 or so) until this unlocks. Remember, it must be 100 UNIQUE waves...you cannot keep playing the same wave over and over getting perfect to unlock this. It will only count as one.