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).
Barik's primary shot is joke from far away, use this to your advantage and force the Barik to either stay in place and try to wait for help or go after you. Be ready for both of these options.
If Barik is running One Man's Scrap on a high level try to use CC (To prevent Barik destroying them himself) and avoid the turrets to prevent this form of healing.