I kind of like how this game doesn't have a double jump. One less thing for the AI to abuse in a way that humans can't.
BTW, games don't need multiplayer to be good. I'm having a ridiculously fun time just killing AIs on Insane and trying to powerlevel my Medic to 50. Besides, if it were multiplayer than it'd just get hacked and that wouldn't be any fun to play.
Anyways, my main class is the Medic, though I've played all of them a bit. My Medic is 35 or 36, my Tank is in the low 20s, my Assassin is 13 or 14, and my Commando is 11.
For the Medic, I ran Magnums until I got the AK-47; that thing is just a beast of a gun no matter how you slice it. You shouldn't even need your secondary TBH.
For the Assassin, I really love the Jackal for its 100% accuracy. Beating people with a baseball bat is fun too, but it gets frustrating when your arm is sticking through the guy, yet your bat didn't do any damage...
For the Commando, I don't really have any advice because other than (almost) never needing to reload, he doesn't have anything that would make him better than a Medic of equal level. Maybe I just haven't leveled him enough yet, but I really don't see a point to using him.
The Tank is my second-favorite class. Shotguns are fun to use, though I'm not a fan of the shields. You have to be point-blank to do anything with them, and at that point you'd be better off blasting their head off with a shotgun. I especially love how the starter shotty's name is the "Defriender."