Cassian is one of the strongest rebel heroes at the time of this post. Because he has a very long range, and his passive ability helps boost the range of other ranged units Cassian is very good at getting damage on enemy turrets before they have much of a chance to react.
Cassian also has access one of the few units in the game that can taunt, K-2SO forces enemy units to attack. This can be extremely powerful on defense, especially because leaders can be trapped by his taunt and subsequently killed.
My decklist is as follows: Cassian, K-2SO, MLC-3 Light Tank, Hyper Adrenal, Aqualish Engineer, Gigoran Rebel, Rebel Pathfinders and Rebel Grenadier.
Other notable things when using this deck, Gigoran rebel can be used against AT-ST's Dewbacks, and MTV-7 Light Vehicles.
Gigoran Rebel should generally be saved to kill one of the things above, or could be used in combination with K-2SO to trap and kill an enemy leader.
The general playstyle of this deck is to sit back and defend enemy attacks until there is a good opportunity to counter attack.
Best of luck to all you dirty Cassian wanna-be's. Go out and kill some Krennic.