If you are used to doing headshots while bullet-jumping, this Riven challenge is very easy - otherwise it may require a few tries. The easy way to unveil this riven is either use a Warframe that is able to stay airborne longer (like Zephyr) - or have some good Crowd-Control that will keep the enemies still while aiming (like Rhino, Nezha and Limbo).

I decided to use Zephyr just because I haven't really used this one for a long time and it is really useful for this challenge..

Whatever warframe you choose I highly recommend that you equip some mods that increase your aim-glide time, e.g. Patagium and Aero Vantage.

Preparations:

• Select your favorite Warframe for this kind of challenge and mod for max aim-glide time
• Equip a sniper rifle or whatever weapon you prefer for making headshots. I used Rubico Prime because this is my favorite sniper rifle for Eidolon hunting so I am very used to this weapon.
• Make sure to not equip any sentinel or companion that may kill enemies before you can do the headshots!
• Don't forget to equip the unveiled Riven mod!

Select a mission where lots of enemies will spawn, e.g. a Defence mission and wait until at least 3 are nearby (I actually prefer waiting until there are 4-5 in case you miss a headshot).

Then charge Zephyr's first ability by holding the 1 key in order to jump high up into the air - aim for the heads. Challenge done!

If you don't have Zephyr you can also use e.g. Rhino or Nezha's 4th ability or use Limbo's 2+4 combo to crowd-control enemies while doing the aim-glide.