Get Horde lit without starting the Barn mode.
Light Barn mode.
Start Barn mode.
Start Horde.
Play Barn forever by using Horde add-a-balls to knock down 3-bank drop targets to add time to Barn.
Barn maxes out at 10M a shot, but this can be multiplied with the 2x riot shot multiplier and the walker hurry up shot.
The game rolls at 4.4B and I hope they have fixed this as of GOT.
Pretty much any of the non-beatable modes are good to stack with Horde. My favorite is Arena + Crossbow Frenzy + Horde.
I’m still on the fence a bit about the MK values resetting after Horde. Seems you need to decide which road you wanna take. Multikills and LMS or Horde. I actually enjoy Horde much more now that I shortened the timer. As you say on factory settings you have plenty of time to pick of weapons targets in the early stages.
Set it up in tournament mode and you don’t have to worry about multikills resetting after Horde 
I prefer Barn because the value doesn’t go down if you fail to hit the shot repetitively within a short time as it does with Arena. As for Riot, I personally find the shot much more difficult than the left loop shot for Barn.
The Woodbury Skillfire award is also good to stack in Josh 