I've now managed to do it twice on the 'Not In Kansas' way, once with four people and once with three. There are advantages to having more people as any Team challenges add 500 points to everyone's score so you lose 1000 if there are only two. With four it was easier to get two people to do the leashing and the other two to run around killing bosses and distracting enemies so the leash two weren't overwhelmed.
I did find it much easier second time round even with one less person because I'd practiced the boosting a lot.
It takes about two hours to get to wave 20 with two of you and you can just replay it if you don't get the 'Anarchy Master' to pop before the last guy, get killed and start again. It is all experience points and wave 20 has twice as many enemies and lots more team challenges so it's a good place to start.