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!
To counter Khan shield without buying wrecker, go to the highest platform and shoot above him. He will resort retreat. There is very rare chance if you manage to shoot his feet just like Vivian. Only snipers can do it but very rare.