4/13 Update: !Challenges are possibly broken for some players!
The Coalition talk about the issue four minutes into their recent stream:https://mixer.com/GearsofWar?vod=97256907Please feel free to down vote this solution if you would like an alert for when I find out this is working again, I won't take it personally.
For those that don't know , TA guide editors can choose to alert down voters that they've updated their guide. The next time I update this guide will be to let people know that it's working again, I wont' spam you.
I'm pretty sure this can be done in any versus mode and in any Horde playlist.
I did Casual AI versus as you get twice the points from versus as opposed to Horde for specifically this Challenge. While I personally didn't try Horde, you probably get points more consistently.
It's your call.
Versus AI took me a little less than seven hours of playing.
If you decide to play on Horde if you decide to play full matches, it should take three runs or if you want to do the "Sniper trick" then you're looking at about a hundred waves. Sniper trick is probably faster but way more tedious.
If you play normal versus it should take roughly three hours, I guess AI matches give less ending match result points.
(Thank you everyone that contributed to the above info.)
If you go AI versus the three best modes are King of the Hill, Team Death Match and Dodgeball.
King of the Hill is obvious, just run around and get kills. If people are ending matches at around five or six minutes you should be getting anywhere around 9,000 to 10,000 if you're a diligent bot murderer. In quick matches you're looking at about 3 or 4% progress for this challenge, if you can hit 15,000 points then you should get 5%.
TDM just do your best to spawn camp.
Dodgeball might be the one that raises eyebrows. It's a very quick mode where rounds should usually end in under 40 seconds. If you're lucky and in an empty lobby, you should be getting all the kills and getting over 2,000 points. So you'll get 2% progress towards this if you can beast in this mode in roughly four minutes. (This is accounting for wait time in between rounds.
If you want to do the Horde runs, see the guide by LORDSONGOKU
If you decide to play against other human players pick Escalation, King of the Hill or Team Death Match modes.
Here's my guide from the last solution if you want to farm bots:
Here are the general highlights if you don't want to read all that:
-Use power weapons.
-Snipers and Torque are great for long range.
-Vote for small maps.
Here's a new tip though. DON'T STEAL KILLS! Stealing a kill from and opponent that you did no damage to only nets you 10 points, it's much better to mark them and let someone else get the kill so that you get a free 30 points. You should generally be marking as much as possible. I think The Coalition is trying to test this Challenge as a way to mitigate some "kill stealing". However I doubt many people would have realized this had I not said anything.
If you did however deal some damage to an opponent that got down, then do kill them to get your points quicker.