Banned how exactly?

Same here - I've upgraded 3 of my 5 chars to lvl 48-52 and found good strategy to make max damage (Ventress - as leader, her contract reward boosting dmg significantly for all chars; Barris - as a damage dealer with her multiple attacks in one turn + significant healing; Chewie - as healer). With that config I've managed to get to the 40% of 7Th level of Coliseum and after I lose that battle - my account got banned for "cheating in Coliseum", but obviously I didn't.

I dare to assume that the developers didn't well tested  the mechanics of the Coliseum mode before release, so the successful selection of a pack of characters is considered as a bug-using, and players are paying for that by losing their accounts.

I've sent a report / ticket and asked for investigation on that issue to make them assured that there wasn't any cheating - waiting for response...