To do this achievement, you need to enter the plains and kill an enemy from at least 100 meters, that is quite far from the warframe standard kill's range.
I recommend to use weapons like Snipers, allowing you to aim with a gunsight and make it easier.
Remember that, if you come too close to a group of enemies, they will spot you and start rushing toward you. To avoid that, stay at distance and use your waypoint to spot the one you want to kill. Doing this way, you will see immediately how far is your target from you.
Be sure that you kill your target while it's at 100 meters far from you or more. Pratically speaking, choose a waepon that has decent damage, to kill it in one shot. If you don't have it, before entering the plains, go to Konzu and a choose a low level bounty. In this way, the enemies will be easier to kill.