I was able to boost this in an hour using a maxxed out ranger. I originally tried riding a graug or caragor, but lining up the enemies so the animations kill them is tedious and time-consuming.
I went to the South Uruk base on the second map, Fort Morn, and played, "What's the biggest combo I can get?"* I jumped over an enemy and used Vault Stun to freeze them, stabbed them in the back 5 times using Critical Strike 2 to triple the value of my hits, and used the
finisher to kill them. I prioritized shield enemies over berserkers over javelin throwers over plain Uruks, and that let me keep the combo going and spam the
finisher to insta-kill enemies. Throw in the occasional bow shot to kill fleeing enemies and a Shadow Strike or two to get enemies who are positioned far away and the kills rack up quickly.
I did not Brand any targets, as I wasn't sure I would get credit for all of their kills on my behalf. I also did not use any branded beasts in case their kills didn't tally either.
I'm on TA frequently so please comment if I need to expand. I give credit to all contributors, so let me know if you find a faster way.
piercem3 said: "I got this using a very similar method, though I mixed it up and used the
stun followed by the
drain. If I was lucky, I could get 7 or 8 kills at once with this, and keep my combo going. I would also suggest using runes that refill your health as much as possible to stay in the fight continuously."
DEMON362 said: "can also use the upgrade shadow strike and you will kill a bunch or uruk in a row, i was getting between 8-10 kills with shadow strike. then i would mark 2 of them to gain arrows back and rinse and repeat. was able to complete the challenge in 45 mins"
Shadowmere142 said: "Branded beast kills don't count. My Caragor killed 8 or 9 uruks and my percentage didn't go up."
darkerbydesign said: "Fire explosions are a good way to gain a lot of kills quickly, in some cases getting around 15-20 if you can get a bunch of them around the fires, grog etc"