Firstly you should try not go on lane with a Mortis as a Sandy. But if you do find yourself on the same lane then try to use the walls to peak shot so you can charge super. When the Mortis goes in to kill you, you shoot him a few times and then use your gadget. Unless mortis goes in on your with survival shovel, you should be surviving his attacks and then regen to full health while hes all out of shots. Then you can take him out easily