Don't face Strix head on. Pouncing head first may result in a (skilled) Strix sniping you up close, if caught in his scope. Sneak behind him and lower his HP with your knives. Once he knows you are there, pounce on him to finish him before he hides.
There are not a lot of things you can do here. Use walls as cover and wait for a flank to kill Nessa is most likely the best solution Using his shield can block a few shots (You can do this if you are really close to capturing the point etc).