@Giovanni Valia The one I use is on the back side of the burning building, located at: -2388.6 239.0 -102.6. As noted with the big red arrow, by standing in between these two patches you take both their damage aura.
I wanted to verify this spot again after I posted, so I did two runs. My average was 2,079,048 damage taken per hour. Therefore, using this method, it would take approximately two days (48 hours) to complete all of the damage taken badges.