I know I'm extremely late but just for the sake of necessity, the mando death trooper lieutenant with their flamethrowers seems to be effective, still can cause up to about 6 casualties. But if you use the mando basic death watch troopers they can tank a number of shots by the gunship before they die. Use them as meat shields and the lieutenants as your buffer, and they don't do friendly damage with their flame so... FLAME ON!!!

For anyone else coming to the thread. Use Rebellion AT-RT walkers, they take 4 hits to kill and there are 8 of them total. Only hit the dropship in the air so reset the walkers target when it is in the air. They do a butt load of damage to the drop ship, and in my experience the best way to handle them is to bait the AI in space or auto resolve with the walkers