10,000,000 damage per hour is comfortably attainable using this method. This is the optimal way to rack up damage. However, it only works on damage and not kills or assists so it would of course make most sense to complete these achievements first (using the top voted solution by Lambent Lail):
Now, you should have about 35-40 million damage left to obtain this achievement, meaning you should be roughly 60-65% complete towards your 100 million damage.
Select the map called AllFathers Arena (It should be the first map when you create a private versus lobby):
You’ll need 2 Xbox’s and at least 3 controllers to make this optimal. If you only have 3 controllers then you can use the 3 controllers on your 2nd Xbox to sign in and select what teams you’ll be on. Then you can connect them to your Xbox with your main profile on it (this will disconnect them from the 2nd Xbox but that’s ok) and again, select the teams. You want to play 5 against 1. That 1 on the other team is one of your guest accounts or the 2nd account on your 2nd Xbox. You will be damaging your team mates.
Game mode: King Of The Hill
Weapon Substitutions: Everything must be changed to Incendiary Grenades
Here’s your KOTH settings (I’ve missed out all irrelevant settings - these are the only important ones):
Fill with Bots: OFF
Rounds to Win: 1 (this will last 25 minutes so you won’t idle out)
ROUND TO SCORE LIMIT: 270
Friendly Fire: REDUCED (NO KILL/DBNO)
You will now stand your 4 team mates bunched together at the location shown in the video and you will grab the closest grenades in a clockwise fashion and throw them at your team mates, maintaining constant damage by spacing out your grenades and keeping the fire burning. I show an example of the timing in my 2 minute video below. You will be getting 1 million damage every 6 minutes if you replicate what I’m doing in the video. Make sure to throw the grenades just in front of the group. The fire spreads forward from where the grenade lands. I knocked out this achievement in 3.5 hours after getting my 100,000 kills and 50,000 assists done by doing this:
Edit: studlybambi says damaging yourself adds more damage. You can find his comment below or in the comments:
“I would like to echo what other people have said about taking damage as well. I was able to get at least 7% every 25 minutes which works out to about 15%-16% an hour with loading times. So it's definitely worth jumping in the fire yourself on the first grenade and then throw the second one and go get some more.”