As a Phoenix leader, Cassian can be easily used anywhere on your list. He doesn't need to lead Phoenix units, but he works well with Jyn Erso and Sabine Wren since they both get extra actions from attacking an enemy unit engaged at melee range. Of course, if you are running Cassian without these two characters, this won't work for you.
Cassian can move during his activation, so feel free to designate one of your troopers as the target of his ability to give him an extra activation if needed. Or just keep moving your snipers or other ranged troopers closer to get them into position before the battle begins next round. Thermal detonators don't care about engagement, so if you can get your units into close range, then they could be in for a surprise.
Covert Ops makes Cassian's command card suite a bit more flexible, giving him access to both a movement and a blue melee reroll depending on what is most helpful for the battle at hand. Remember that Co-Ops does not function with Sniper, and it goes away if you perform a melee attack.
Battlefield Leadership is the best overall of his command cards, giving him three rerolls at focus/evade for ranged units on top of the usual tactics action. This means he'd be able to take two shots and give those units up to six rerolls as long as they were within Range 1-3! Cassian does have other very nice command cards, though:
Sniper is almost as good as Battlefield leadership but requires one more action for just four focus/evade rerolls instead of three, which can hurt sometimes. It's only better than Battle Leader when you ranged units outside of range 3 or are engaged in melee, but it's still a very good card. You can either double down on Sniper or go for some lucky rerolls with Pinpointing, making Cassian one of the only Phoenix characters to get more than two focus/to evade results. You can use this with Sniper and Battle Leader since they don't specify that you have to take action to activate them. This command card is very useful if your opponent didn't bring many vehicles or none at all.
Finally, Cassian's Targeting Network (3) is the only command card that does not use an action. Still, it will most likely be used as a blue melee reroll instead since snipers have nine health and three wounds, which means they won't be going down from a single melee attack unless their opponent has a strong ability to increase his chances of hitting. Command cards should be carefully chosen based on your list and your opponent's list, so we can't say what other useful options might exist. My first choice would probably be Battle Leader if running with Jyn or Sabine since you would get two extra actions every round at focus/evade for just one more point than Sniper costs.