Cycler Rifle is unlock at rank 28,it cost 3100 credits.
I actually get the achievement in the Blast Mode (Team Deathmatch),
My method is using Trait Cards Scout to disappear from the radar and give your enemy a surprise attack,
because the Cycler Rifle is not a good long-range weapon,
You will need more time to aim and get the headshot.
(Trait Cards Scout is available at rank 15,Cost:7000 credits)