I also ended up beating it today - came down to the wire - it was the only one I was close on. Rex > Ahsoka > JC. Needed every target lock and DOT to land.
I'm out of tin foil, but I honestly think I had better "RNG" the crappier my reinforcements were. I put Sarge in my line up (very weak ship for me) as the last reserve, and DOTs seemed to stick, and TLs didn't get resisted. Of course, that might have been because I targeted a Clone Sargeant instead of Rex. See, Rex was always front row, left - every single fight. I automatically targeted that spot and didn't even notice what type of ship it was. Later on in the fight I noticed a blue ARC-170 and sure enough, there was Rex (back row, right). I can't remember if I had killed that one or if it just reinforced Rex in? At any rate, the fight I won was definitely different in nature than the many that I lost, despite having the same fleet. ¯\_(ツ)_/¯
And whoever said the Clone Sarge ships hit softly as did the Mace capital ship....they each crit for over 30k. Maybe that's soft to you, but not for me. Overly difficult challenge.