If you have gotten 3 stars for all missions let me know what happens. Otherwise, seems the primary goal should be to unlock the shock trooper as that is a permanent unlock for all future battles, as opposed to getting some temporary benefit.
My suggestion is focus on getting at least one star on each level. Easiest thing might be to simply focus fire on the HQ as it is often defended by only a couple of canons. Taking out the HQ will guarantee you at least one star so you can move on to the next mission.
Once you get to the end (mission 26) you will be able to reply this mission over and over again, each time gaining at least some points. This way, you can get to the 6000 points needed for the shock trooper by brute force.
As of this post you have a little over 9hrs left to complete this.