Redo your mods. Looking at your clones (which are your best team), you should have +100 speed on all of them. Arrows should all be speed mods. Don’t level up any mods that don’t have a speed secondary or for arrows primary. For your arena team, mods should have a minimum speed in the teens, 20’s would be better (though hard to get early game). If you average +15 each, though, with +30 on the arrow that’s +105 speed, which is decent in the early game. Mid-late game you’ll want +120 or more.
For toon specific mods, Fives passes on offense, health, protection and speed when he sacrifices himself, so his triangle and cross mods should be one of those categories (offense is usually the best choice). I would also probably give him either a speed or offense set, as your other 501st will have plenty of health after he dies. Rex should be all speed. Echo can be either offense or critical damage. Until you gear your arc trooper better he should be speed as well, since he relies on Fives’ stat boost to have any damage output. Shaak Ti can be a speed/health (or tenacity) set with protection primaries on the cross/circle/triangle.