Ok so obviously this is going to be easiest with a sniper but before the recent changes to snipers with the Plains update snipers sucked so most people don't have one.
Thankfully DE has you covered!
Go to https://www.warframe.com/promocode
Log in with your Xbox Live account and input the following code for a free Vectis and Vectis skin.
While you're at it put in the code FREESWORD for a free Heat Sword.
Once you have the sniper go to the plains and head west from the door. About 100 meters in there's a rise with a lake below it. This lake has a bunch of Grineer patrolling it. If you go straight to the plains from orbit these will all be level 5-10 and you should one shot them with your shiny new Vectis.
When you're zoomed in with a sniper and you put your crosshair over an enemy it displays how far away the enemy is so make sure it's over 100m away. One final tip, make sure to use the secondary fire button to zoom in fully as this gives you a damage bonus.