The Scout class in Multiplayer has a bolt-action rifle unlocked by default, so kill an enemy scout with it and you'll earn this achievement!
I got this achievement after shooting an enemy and a teammate finished them off before they had a chance to heal. The default Scout rifle seems to do 75 damage by default so shooting someone and your teammate killing them should give you the Assist Counts as Kill points and the Achievement.
I tried to look for the glint of the enemy Scout's sniper rifle scope but I haven't been able to see it consistently.
If the enemy Scout has been Spotted look out for the Crosshair emblem (it looks like a Plus sign with a circle around it, with the top of the plus missing). You can see it here in the upper-right corner of this image:
According to
TorqueOTheDevil in the Comments there is another method for getting this achievement: While on horseback you should be able to get a Scout kill with the rifle and earn the Achievement as well.