Some fighters seek to master the use of  archery to better strike down their foes from afar. Those who choose to train in Keen Archery learn specialized aiming techniques for use against those they have chosen to kill.

Take Aim

Starting at 3rd level, you can use your Bonus Action to focus your attention on a specific enemy you can see. While doing so, whenever you deal damage to the target
with a Weapon using your Dexterity modifier, you deal an extra 1d6 damage to it. This effect ends if you lose concentration (as if concentrating on a spell), lose sight of the enemy
or if you attack another target. At 15th level, the extra damage increases to 1d8.

Well Hidden

Also starting at 3rd level, you gain proficiency in the Stealth skill (if not already) and when you are hidden from a creature and miss it with a ranged weapon attack,
making the attack doesn't reveal your position.

Sharp Mind

Starting at 7th level, when the enemy you are focusing on moves or another enemy moves within 5 feet of that enemy, you can use your reaction to make an attack of opportunity against it. If you attack another enemy than the enemy you were focusing on with this feature, you do not lose focus and can also decide to switch focus to that other enemy instead
(no action required).

Stunning Shot

Starting at 10th level, when you make a Ranged Attack using the Take Aim or Sharp Mind features, you choose to turn that Attack into a Stunning Shot. If you do so and hit that enemy,
it must succeed on a Constitution save (DC equal to 8 + your proficiency modifier + your Dexterity modifier) or be stunned until the end of its next turn. You can use this feature a number of times equal to your proficiency modifier. You regain expended uses when you finish a Long Rest.    

Multiple Focus

Starting at 15th level, you can focus on more than one enemy at a time using your Take Aim feature. To do so, all enemies must be within 30 feet of each other.
In addition the first time you would make an attack of opportunity with Sharp Mind each round, you can do so without using your reaction.  

Deadly Volley

Starting at 18th level, while your are focusing on one or more enemy, you can use your Bonus Action to deepen your aim. If you do so, all Attacks you make this turn that hit an enemy you are focusing on, become Critical Hits regardless of the roll result. At the End of the turn you use this feature, you gain 2 levels of Exhaustion. Once you use this feature, you can't do so until your finish a Long Rest.