Does anyone know the fastest way to level up in too human?

You have a combo meter on the left side of your screen. You'll notice it build up when you kill opponents. The higher that meter is the faster you're gaining experience. It also shows the status of the Ruiner(R2) and Battle Cry(X) attack you have. If you don't use the ruiner or battle cry you'll level up faster but sometimes you have to use them to stay alive. Playing online builds up your experience faster too, but you don't get as many credits.

I find that playing co-op with someone a few levels higher than me (no more than 5 though) levels me up quickly.

Kill stuff

Get in gear with maxed out exp runes 15% get whatever u can for combo meter easiest if u have a co op partner use sentient wep and u then proceed to hit something so u can get combo points from it. do this in 3-4 at the spider farm fastest exp around

Along with having maxed combo meter, you can go through hall of heroes repeatedly. Monsters level with you, so you will probably want to beat the game once through to unlock achievements. Another common training area is world serpent section 4 because of "Spider Farming"