The last campaign and beyond is quite difficult because of the sheer number of death troopers. I was able to get 1 star after barely taking down the HQ in 26 and barely 2 stars in 25. There are a couple of strategies I will be testing out and will let you know how well they work. The first 2 strategies should be able to get you at least 1-2 stars. I think the 3rd strategy is how Disney intended us to get 3 stars. If anyone has any other ideas, please post below.
1. Mass Dewbacks, which does area damage to take out the death troopers faster. However, this might not leave much DPS left for taking out the buildings and turrets.
2. Mass soldiers and storm troopers, which have high DPS to take down all the death troopers as quickly as possible, while having bare minimum of DT/tanks required to take down turrets and the rest.
3. (This might what Disney intended) Purchase the "limited demo" level 3 air strike starship for only 10 points. These will help take out groups of death troopers. Then use your normal composition for the rest of your army to take out their base. As long as you have enough repair droids to survive the initial waves of death troopers, this strategy can lead you to 3 stars and easily make back the 10 points spent purchasing the striker starships.
Campaign The Sickness 25http://youtu.be/rVtyJ8QJrUU
Campaign The Sickness 26.1http://youtu.be/7UU950kFn5A
You can play this campaign up to 140 times. If you are having trouble with other missions, you can milk this mission for at least one star until you get to 6000 points. Simply focus on taking down the HQ using the strategies described above will gain you 90 points.