This has probably already been mentioned, I just found an easy (really easy actually) way to level up, you also get money from it and items/materials, best part you can do it for as long as you want, well, or at least till your fingers can't take it anymore.
I dont know the boss's name, POSSIBLE BOSS SPOILER
It's in the place of the naked android like cloning boss, after you defeat it and do what you gotta do, if you go back there there's a seemingly infinite number of machines that just keep spawning, not talking about one, two or three at a time, but a bugger load at a time, both the huge ones and normal size ones, decent exp from it too. just figured id share it for the new players such as myself.
If you think I care then you're mistaken!