According to the wiki there's 100 levels per rank, with ten ranks total. There's some serious grind in this game, I'd be surprised if I get to 100. I think I'm at 37 now.
When I first started I mainly used the Dawn patrol. I also really abused the motion sensor and targeting goggles. Seems like people rarely use either anymore. The Bear-Stalker was also used alot initially, but that's also died down a bit.
My Current most used Class:
The Deep Freezer pretty inconsistent, but I'm getting better with it. It's fun to use when your doing well, you can really rack up points with it.
I hope they do something about that sword though, that plus ninja smoke bomb can be frustrating to play against.