I didn't really pay attention to damage. But my characters were taking a beating once in a while and Chopper and Kanan both got taken out, although luckily they were revived by Hera's Backup Plan ability since I had given both of them that. But I finally got through the final tier and got 3 stars.
All my characters were gear 9 and 10 too. And Chopper REALLY is needed for the final tier. Last time I was using Sabine and couldn't get through it. I then finally got Chopper to 7 stars and maxed out ALL abilities and got him to gear 9. He made a HUGE difference.
So maybe gear up the characters a little more and use Chopper. That should help survive the final tier. Also mods will help, but I didn't have my Phoenix characters modded correctly but still got through the final tier. With Ezra, always call in Zeb or Kanan to assist because of their protection gain. And try to use Zeb and Kanan's protection gain abilities whenever they are available and use them on whoever is low on protection or doesn't have protection. That should keep characters around a little longer. Plus go after the Death Troopers first and as soon as you can. I know that one character starts off with Taunt. Just keep hitting him until Taunt expires or Ezra can get rid of it with his one ability. Then start attacking the Death Troopers. That's until the one character gets Taunt back again.
The tier isn't impossible either. Because I used Chopper instead of Sabine, it only took my one try this time to get through the tier. Also, you could just hit Auto Basic until the final wave/round. That's another thing I did.