Arena advice please

I'm going to make a suggestion a little bit different to what has been said here. Unless you're planning on spending a bit of money then Rex is off limits as he's only farmable from chromiums and crazy crystal amounts from shipments. I'm thinking two important characters that will benefit hugely in the long run are Lando and STHan. They're both scoundrels and will allow you to complete high tiers of the credit heist event. They're both also rebels and if you farm wedge and Biggs who are the best 1 - 2 combination in the game at this point in time. Under Lando's lead they gain crit damage and speed. Wedge is a tough farm though and with Lando can only be farmed from cantina energy which his why I've put off going for wedge for so long now but I'm finally starting now, far later than I should have. A decent toon to round out this team might be somebody like Anakin. He's also a tough farm gear wise but seriously worth it in the end, and is a Jedi so will go towards getting Yoda for yourself.
It would be a bold move to drop your current team and adopt this whole new team and would probably hinder your progress in the short term but would be incredible for your long term progress

Edit - sorry I didn't realise you already had Rex