This simply means killing one enemy with a ranged weapons.Impossible to miss as the third of Gromar's trials has you hunt deer with a bow.
With a bow equipped you need to hold because just tapping it won't do anything.Then you'll see 3 orbs filling below your crosshair, the more of them are full the more damage your shot will cause.
Crossbows work a bit differently, it'll take longer to put a bolt into the weapon but it will fire immediately afterwards and always cause the same amount of damage.
Both weapons count for the ranged achievements but bolts are harder to come by.
Of course you can also weaken your enemies any other way but the killing blow will have to come from a bow or crossbow.
For the first 2 ranged achievements you can use to auto aim, however the third one is more complex(see below).