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Credential's: 450+ hours in Nightmare running various builds.
Marksman will always be in the back of the group, furthest from the action though perhaps is the most important in terms of mutation damage. Since you'll consistently be scoped you'll often see mutations prior to your team, also if your team calls out a mutation you tagging it with "Marked For Death" shows you're aware and prepping to snipe it. This additionally amps your team's damage by 40% against your marked mutation. Additionally "Patient Hunter" being a ramping 10%/20%/30% damage boost should give you ample amount of damage to get through the early game. From there it's your standard weakspot and Stock Pouch for ammunition and base damage. Now just because you're in the back doesn't mean you're immune to the threats of being overwhelmed by the ridden and mutations. Having just Run Like Hell and Mad Dash permits you the ability to relocate to a safer area prior to aiming down your sights again. If you have ever played sniper in any PvP game you'll know how important relocation is. Overwatch is just to provide more utility as it is a team game and anything you can do to contribute will significantly help out. Also Glass Cannon is an auto include if you're good at repositioning and avoiding damage 90% of the time.
P.S. Since you do have the movement speed from Mad Dash + Run Like Hell you will be required to constantly review your surroundings to ensure your own safety. The 30% damage amp from Patient Hunter is not always worth it, and you may be better off using it as a 10% or 20% boost to damage instead. You are REQUIRED to move prior to ridden closing in on your location if you're in danger. If you feel like you're about to get hit, melee them away PRIOR to sprinting. Building this habit is very important especially since late game you're wanting to take Reckless to offset the sprint efficiency consumption of Mad Dash + Large Caliber Rounds.
