Fastest way to do this is to load up a custom escape map (Escape > Map sharing > 100K Farm > Host with this map)
I made the custom map "100K Farm" ID: SCHF51. It's 12 bays of 200 Juvies each. The juvies are set to aggressive for speed. Enjoy.
You need to kill enemies in Cole's Ultimate ability with fire skill cards on. Killing enemies get your Ultimate back faster so just keep killing as it's an unlimited respawn. Kills that "burn out" seem to count. So if you see the flame bleedout symbol next to the name it counts. Took me about 10 mins to get.
This is the card setup I used (most important is "Killing time" as this will extend it every kill):
gonna leave my previous solution in case TC decides to patch custom maps