Did this at the very beginning of Act 3 Chapter 1. You need a support unit with teamwork 2 and empowered skills unlocked. There will be a group of 3 tickers closely grouped to the right of where you spawn. I empowered Mikayla and used teamwork 2 and then sniped the tickers. You must use Mikayla and Gabe for this mission, so If Gabe has the skills unlocked you don't even need to worry about your other two units.