If you're going to do the score path, make sure you have the med kit. It gets crazy, and after the mouth area you still have to deal with the final area where all hell breaks loose. I got through with the 20% of health left, so I felt really good when I got through the gate and the trophy popped. Shocker/Mines/Flares, and RUN LIKE HELL!
EDIT: I made it to level 10! I took a risk and went for the money route on Round 7, as I still had my medkit from Round 5. Unfortunately I accidently used it way too early and had to really work my way around 7 and 8 without a backup. Somehow I did it, and got to Round 9, which was of course another health run. Got a new medkit, but didn't really need it. I aced the final area, and now I'm at Round 10, trophy done, and now to see how far I can go forward.
EDIT 2: Woo Hoo! I got my co-op trophy from a good random. 100% on Dead Nation again!