Echoing what others have said it’s your mods that are holding you back. Speed, speed, and more speed. Consider this: I’m on a late November 2017 shard and here are my speeds:
Bb8 +125
JTR + 111
R2 +108
Thrawn +105
GK +92
Arena is all about turn order and going first so to be successful nearly every character aside from a few outliers (like Resistance Trooper) need max speed. Going first dictates the entire fight. You can stun, fracture, remove tm, etc. before your opponent gets to do anything and you gain a massive, usually insurmountable advantage. Almost every character should have a speed primary arrow and all your other toons should have as much speed as possible on secondaries. For your arena squad your speed secondaries should all be in the double digits for the most part.