I've seen a lot of EXP/SUPPLY guides from many people that are good, however, the method that I have been using has gotten me 1-2 ranks up since I've been doing it. So basically, what I do is make a BG: Easy mode room. Sounds to simple right? Exactly! It is simple all you got to do is make a Bg: easy room and play it thoroughly and beat it and the results are just incredible.
Results: 500-999 exp (excluding boosters) by default. The reason that score is default is because of the kills you earn in the game + the bonuses such as lucky shots if you manage to get them.
I also get 5k suppy included with the EXP. *I've been noticing some people have issues with the 5K supply, I'll see what's going on with that as I have no problem with earning 5k supply.
If you have boosters then better for you because you'd be hitting those 1k exp and 5k supply points like nothing. I will provide screenshots and video proof to show you guys that I'm not BSing. I hope this guide has helped you and enjoy the extra EXP/SUPPLY!
*This is not a hack method. So if you plan on hacking then you might as well uninstall.*
Video proof: http://www.youtube.com/watch?v=piE1ewtj2rg
Screenshot proof: http://img849.imageshack.us/img849/971/ava13010219435800.jpg
*UPDATED VIDEO* (Tutorial for BG: EXTREME SOLO)
^The video above includes tips/tricks to beat BG: Extreme by yourself. So if you want to try it make sure to watch the video above to gain some insight on how to win!