Try to burst Zhin. When he uses Counter, run to a safe distance, then open fire like hell. Additionally, his attacks are very inconsistent, so if you are a character with a burst type weapon, you will usually win.
Slither cancels Hunter's Mark. Giving your position as a healer is bad, so it may be a good idea to slither away to get rid of it. You can still heal your teammates from a distance!
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.