how do u get to create some missions?

some people might agree with me some might not but the why is it for everything else u just have to destroy 30 percent of their fleet but when u attack the reavers you have to comepletely destroy the armada and when i actually do get them to leave they just reappeared about a min later while i was mining someone help me out with a hope for a little more fairness for attacking the reavers.