Use the first level (A-1) and turn on the Hollow and Black Eye skulls. This will give you a 4.5x multiplier, meaning you get 900 XP if you get gold and 450 XP if you get silver.
I got gold about 2/3 of the time without even trying. With menu and load time, it's almost 2 minutes per round, putting you at 25k in no time.
The Hollow skull means you only have a shield, so you can't go too crazy. Even then, I only died 3-4 times in 40-50 runs through the level. If you really wanted you could try to get your shield back my using melee (that's what Black Eye does), but that will probably cause more deaths than it saves since you get shot pretty quick.
Good tip in a comment from Darkened Night. If you do end up with low shields, you have a Regeneration Field as your AA. I had actually forgotten all about that while I was playing.