How I did this quickly:
In a private lobby:
Juvie Madness (on the start button and scroll over to Juvie Madness on the enemy list section),
Scout, using the following cards:
Lvl 6 Health, Lvl 6 Health Regen, Lvl 6 Brawler, Lvl 6 Shotgun Damage, and Lvl 5 Shotgun Capacity.
At Wave 1, I put the Fabricator back at statue and then used the Gnasher and melee to kill Juvies and Trackers. Energy you collect can be put into decoys, barriers, sentries etc., to get the Fabricator to level 2 to get the Overkill from the Fabricator.
Don't worry if you dont have the SP, it can be done on any map, going back into spawn or running around if you so wish.
Thought this was a little more engaging rather than doing a straight up speedrun. Things can get chaotic, but if you have decent level cards, it shouldn't really be a problem unless you get multiple Dropshot Scions on Boss waves. I did this in around 2-ish hours.
EDIT: You can check your progress here: https://gearsofwar.com/roadtogears5
Thanks to @plesniaczek in the comments for pointing this out.