I think Cassian is really underrated. He is able to debuff a lot, makes good damages, has synergy with K2. I used him yesterday in Heroic Rancor, I could solo P1 and 50% of P2 with not so good gears and mods.
I used Teebo gr 9, Cassian gr 9, TFP gr 10, QGJ gr 11 and Rex gr 10. As soon as I can, I will replace QGJ by Jyn and this team will be great. Also need better mods especially speed for Teebo.
Cassian gives Speed down AND offense down (also buff immunity and ability block but only useful with the green pigs).