Finally got this, and I agree with everything that ScorchPSO says:
- Definitely disarm (
) the shield and electric baton guys instead of using your combo to take them down. Guys with clubs don't really matter. If you run out of people to disarm THEN use your normal takedown (
- Spam the grappling hook takedown (
) as a way to get your combo up for takedowns or disarming. Don't hold left trigger too long or it'll trigger something different.
- Spam the dodge button (
) as well, you can usually dodge twice without losing your combo, but stick to one if you can.
- During the disarming, takedown, or grappling hook animations take the time to gather your thoughts, refocus, and plan your next move.
I didn't pay too much attention to the guns, if you can't disarm them then just kick the guys or tap
real quick and that will stagger them for plenty of time. I also think it's possible to get the score you need even if you get hit as long as you have a decent head start (~500K), but your second half will also need to be all in one combo. If you get hit twice then yeah your chances are slim-to-none, but even then you still may want to keep going so you can practice on the later waves of enemies.
The achievement will pop when you hit the last co-op takedown to kill Croc, but enemies will continue to spawn as the room is technically endless.
It still took me several hours, but here's the video of when I finally got it (you can see I hit 1 mil at around 394, but at about 300 you semi-regularly get 10K for every 1 guy you take down):