Squad Arena Store farms, in order:1. Greedo => 5*
2. StormTrooper Han => 5*
3. Leia => 5*
4. Ackbar => 6*
5. STHan => 6*
6. Leia => 6*
7. Ackbar => 7*
8. Leia => 7*
9. STHan => 7*
10. Greedo => 7*
11. IG-88 => 7*
12. Cassian => 7*
13. Ventress => 5*
14. HK-47 => 5*
15. Sidius => 5*
Note that the last 3 toons are "Just in case" at this point. If your NightSisters need Ventress before that, though, that's fine. Because I used so many intermediate steps, this looks like a lot more work than it is. The first 10 steps are just 4 toons.
When we're not working on our original list of 17 characters, we fall back on other priorities, like improving performance in Territory Battles (which for you means rebels generally, but Rogue 1 & Phoenix especially) and bring up your Bounty Hunter squad for use in the Bounty Hunter events (there are a number of them) and the BH mission in dark side Hoth TB. If there were Imperial Troopers in the Squad Arena store I would have added them to this list before Ventress, since they, also, have special missions exclusive to them in dark side Hoth TB. In any case, I'm trying to balance those other priorities when we get past the 17 characters we know are tops.
In any case, this is a good starting point and if you want to vary things up later, that's fine.
