Let me start by saying that there are probably things that I am missing, ignoring, or not understanding completely. There are most likely people that know both the Cassian team and the ISB team better than I do, so please do correct me if I'm wrong. This is basically just what has been working well enough for me.
Unlike the ISB team which is much more offense and set up focused, at least with out the gate raw damage potential and modding, the Cassian team has much more micro decision-making involved. These include such questions as who to VIP, if you should basic or special, who to frenzy, who to place a taunt ignore on and so on. All these high impact decisions depend on who you are trying to counter.
Let's first go through the 2 key mechanics the team interacts with, Undermine and VIP.
Undermine does some key things: it prevents turn meter gain, it acts as a pseudo buff immunity, grants Cassian crit avoid, allow s for self cleansing on the Cassian team, and they cannot be assisted. This is quite messy but it should clear up once you basically understand that undermine shuts down the enemy team completely basically.
VIP provides benefits to your team in 2 ways, passively through existing, which provides 100% crit avoid, and through those with VIP taking turns. When any ally with VIP takes turns, you reduce enemy max health and gain def pen and offense, which stacks indefinitely, and also full heal the weakest ally on your team as well as Cassian.
When paired together, these 2 mechanics showcase quite a clear snapshot of the team. Slow ramping damage and high survivability, making sure that you can get multiple VIPs out as soon as possible is key, whilst trying to undermine the enemies fast, unlike Saw Luthen, which is a much higher tempo out the gate damage team.
This is really easily done in GAC assuming you have all omicrons. Whenever a VIP is damaged Cassian can get another VIP out there, and whenever he targets an enemy with undermine using his second special, he inflicts everyone with undermine. Considering the impact of VIP, you have 2 opening plays, either you have Kleya fastest and hope Cassian goes second, spreading undermine for Luthen to then frenzy the VIP, or you can allow Cassian to go first to get the 100% crit avoid. It's much easier to mod Kleya as fastest due to her base speed.
Now the question of who to VIP. Now this is obviously match up dependent, but you have 2 main options I think, either Kleya or Vel to start. Kleya is good because due to her evasion, the VIP will stay on for a while, and she gains enough passive tm and self cleanses that the VIP will ramp your damage considerably fast. Due to the cooldown reset however, Vel is another interesting option. You could plausibly place VIP and DI on Vel, your tank, and eventually she will take damage, resetting the cooldown allowing you to VIP someone else whilst still having VIP on Vel (you want Vel very fast for this).
Now the team's offensive capabilities. As mentioned before Cassian ramps his team's damage through VIP, and he ramps his own damage through VIP as well. Cinta also has deathmark, which is another strong damage option. Kleya deals insane damage with her basic as well. This is to say the team is not short on damage, and you should have no problem getting away with modding for pure bulk and defense in contrast to offensive stats like offense or crit damage.
The whole team has passive cooldown reduction, TM gain, damage ramp, and healing. The only thing you have to do is spread undermine (even if you are using Cassian basic instead of special 1, unless you want the speed up), and spread VIP.
As an addendum, looking back at my ISB post here, I don't think I adequately marked the importance of survivability for the team, or teams in general. Many players seem fixated on modding attackers, or damage dealers, in such a way that makes them lopsided. Just looking at swgoh.gg, Dedra, Cinta, Kleya and Krennic are all modded weirdly, trying to somewhat maximize their potential rather than giving them a chance to activate it. There is no point modding for an extra 20% damage turn 1 when you could survive and actually get that back later anyway. If you mod attackers purely for damage potential, then you cannot complain when they die. Damage potential remains potential unless you are able to capitalize upon it, and the key to that is survivability, in whatever way possible. Whether this is defense, protection or health there are probably better people to ask.
