There is a earlier way to get the red stars, I am not sure if this has been posted already, but if not, they should be a sticky, as I imagine more and more people will start to realize how difficult it is to get certain stars.
This might sound somewhat like cheating, but I do it when I have played that certain battle/chapter over 5-10 times on hard.
After beating the chapter/saga (I forgot which), there is always 3 conditions to earn the stars on each chapter, one is always beat it on hard, the other 2 varies. I was under the impression that all 3 conditions will have to be met playing on hard, but that's not the case, you just have to make sure you win.
The easy way is to just beat the chapter on hard in whatever way you can (that shouldn't that hard), but with the other 2 conditions, just go play on easy. I have tested it on 2 chapters, and I got the stars for the conditions that I didn't meet playing on hard (little damage….etc)
Hope this helps all that is having a tough time getting stars.
By the way, there is a lot of stuff in the star shop to buy, pretty much I think you will need all the stars completed to buy them all, so this will become helpful down the road.