Kind of stumbled upon this while playing this part on my insane run, you earn a crazy amount of points by playing through chapter 1 of Act 3. You'll need 7.5 mill for the onyx medal, or you could just complete the campaign on insane, but this is not only quicker, but also a ton more fun as playing solo insane is beyond frustrating.  I've put a couple of improvements to the video in the description as I found them out later, my best score for this section is 704,170 and it took about 20 minutes.  
 
http://www.youtube.com/watch?v=j8vP0Nl112U 
 
Hope this helps out anyone that wants the achievement. 

Gears of War is, something I go fucking crazy for

*throws down pen*

Thanks, I already got an Onyx for interacting with objects by pressing X by the Thrashball machine 4000 times, but I'll do this too

This is an excellent tip. Thanks, duder!

Thanks, I already got an Onyx for interacting with objects by pressing X by the Thrashball machine 4000 times, but I'll do this too

Thanks for this. Being using it to get that Force Multiplier achievement as well.

Will be sure to try this, thanks

@RandomHero666 said:

Thanks, I already got an Onyx for interacting with objects by pressing X by the Thrashball machine 4000 times, but I'll do this too

Holy crap, that is hardcore farming.