What is your arena team, Palpatine, Vader, Tarkin, Stormtrooper, Maul?
Why is Vader's lead zeta'd when Palpatine should be leading?
The classic starting lineup for Imperial command squad is Palpatine, Vader, Tarkin, Tie Pilot, Thrawn. Overwhelm the enemy with debuffs and AoEs. Sith, notably Sion / Nihilus can be swapped in as desired.
Your modding is bad: incomplete sets, lacking potency where appropriate and all the speed of a stunned snail standing in superglue. You need to address that.
Finish your Phoenix squad to 7* Thrawn next time and in the meantime see if you can compensate with more gear levels and better modding to make him arena viable.