Try to separate him from his turrets and wait out his shield because of the short duration of the shield. Do not let him be first on point and set up turrets.
Using Nimble as an Item is good against Cassie. The player will have to review his/her shots trajectory to make sure to hit you with your different movement speed.
In the start of a round, go near the objective but stay behind and wait for around 5 seconds, then use Scout to determine which way Skye will flank.
This also directly counters Stealth.