I started off unlocking it and having a hell of a time beating it (like, over an hour of play a day to do it twice). That gradually got easier as I leveled my ships further, and got my pilots with higher gear/stars. Until today RNGesus smiled on me and the AI decided:
1. Using JC special attack was for pussies, and basic'd (or topped up the protection of a barely damaged ally.
2. Reinforcements were for pussies, so keep spamming pink aoe like it's the best thing ever.
3. Spread damage was best damage. I had 4 ships in yellow by the time the battle was over.
The way I always did the battle was: kill Wedge before JC calls him to assist (if you don't do enough damage, then start over), keep a dispell for taunt, and especially for taunt + dmg immunity. Kill both JCs second. Then whatever reinforced, then Rex, and finally Clone Sergeant. Sometimes I killed clone sergeant before Rex if he got taunt cast on him, and I happened to do a lot of damage before dispelling.
Today when I 3*'d it, clone sergeant was down to a red sliver of health and Mace had a turn. Instead of reinforcing and killing my Sun Fac (the lowest of my ships), he spammed pink crap at me with none of my ships target locked. And now I will never have to do the battle again. Easy sims to zeta! :)