This Riven challenge is rather easy but may still take some time to unveil because it involves a lot of moving around to make sure you are 100+ meters away before you take the headshot. 100 m is actually much longer than you might think! I prefer to do this challenge on the Plains of Eidolon bt you can also go to Orb Vallis if you prefer; there are also several missions where it should be possible to get some headshots from 100m away but I highly recommend one of the open worlds as that is much faster.

Most important for this challenge is a sniper rifle with a powerful zoom so you can land those headshots from 100m away:  Both Lanka and Vulkar have a 8x zoom, which is very useful for this challenge. I used a Vectis Prime because I like the fast reload but it only has 6x zoom - for this reason I equipped the Eagle Eye mod which results in a nice 8,4x zoom.

You can use whatever Warframe you like but it is much easier/faster to do if you are able to remain undetected by enemies. I have done this Riven challenge several times now and in my experience Loki + the Itzal archwing + Vectis Prime is my favorite setup for this challenge!

Loki can stay cloaked for a long time and is able to move quickly around while cloaked. Also, the Hushed Invisibility augment mod allow you to fire your sniper rifle without alerting the the enemies, which makes it much easier to keep them at that 100m distance!

If you have the Itzal Archwing quickly fly to the nearest camp and look for enemies - move at least 100m away (change range by zooming with your sniper) and then use the #2 Penumbra ability to cloak - drop an Energy Restore if needed while you remain stationary and cloaked.

Remember to equip the Riven mod and also equip some Energy Restore on your gear wheel to replenish your energy. Switch to solo mode and head to Plains of Eidolon - there should be plenty of enemies near the various Grineer camps.