DeletedUser100832
In order to level up an Arc to 80 in 2 months without spending any diamonds you need some friends who do spend diamonds and work intensely together with you sniping high level Arcs and other GBs by dual/triple jumping so you can sit in their slipstream on the way to lvl 80. Not many players are in the necessary environment to be able to do that. But it's definitely possible.
As for the post above, the ranking system will be overhauled soon and those people will be kicked down to a more normal rank soon. A lvl 80+ Arc can not compete with an active fighter for ranking points at all.
don't really think you need diamonds. They help, for sure. But yeah, the most important thing is either (i) a committed guild of experienced players or (ii) a lot of time to spam people with offers of goods for fp. Of course, that's just to get over the initial hurdle of getting 3-4 of you to about level 25, after that's it's plain sailing, you barely need to lift a finger.