No private servers exist, to my knowledge at least. You can do the usual 'spend 4 hours on ******s with your annoying neighbor's contact info' for free points, but, if you're not a guardian, get the Guardian upgrade before you get z-tokens. Guardian gear makes the game much easier, and with a solid stat build [you can find guides online if you need a good build] you'll be able to complete pretty much anything that isn't an 'impossible' challenge fight.