There are lots of ways to get leveled up quicker … some more cheat-ish than others… but the best way is to just play the game and kill everything and do all the sidequests along the way.
It really is a fun game to just play through all the characters and play through levels.
That is also the best way to get a feel for the various abilities and build strategies.
Don’t take away your enjoyment by rushing it. It will all be over too soon anyways … well if say 200+ hours is too soon … and it was … I kinda miss it.