Just a couple comment on ships and challenges:
- Slave I and/or Scimitar are just too slow to farm, you have to farm another ship to 5*, I chose Resistance Pilot. Maybe it's different if you can't beat the challenges anyway (see below)
- Ship rarity is not all that important for strength, look at the factors involved. I had trouble to beat Home I 5* challenge, but in the end got it beaten, Tarkin's ship was easier to get to 5* (probably because I have TIE Advanced). Zeta farm challenge is even harder, it's important to level up Tarkin's ship and have your pilots at high stars, levels (each gear piece counts), gears and abilities (mods to a lesser degree too). Just as a measuring stick for you, my zeta pilots are Tarkin (7*, g9.2, l80), Biggs (7*, g10.2, l84), Wedge (7*, g10.1, l84), TFP (7*, g10.4, l84), GS (7*, g10.3, l83), JC (6*, g9.4, l73), zeta bench is Ahsoka (7*, g8.5, l74), Sun Fac (6*, g8.3, l74), Resistance Pilot (6*, g8.1, l75). Bench is not that important since the ultimate goal is to 3* the challenge which I did after levelling up Tarkin's ship to level 60, and it was a close and very lucky fight.