That ones abit tricky as people seem to unlock them easy on different levels. I personally farmed the level that you fight super Saiyan god goku which is parallel quest 47 I belive you'll Find a patrol guy in the second area that will drop a dragon ball if your lucky enough to get him to appear. Best mentor is Vegeta. He has z soul that powers up KI Super Attacks biiiig time and having him as your mentor and using final flash which you get by completing his training makes you deadly if your a super Saiyan as you can spam it doing way more damage. Goku is unlocked at 70 then beerus after goku. By power if you mean attributes, Working on health, stamina, KI & KI blast supers will mak you strong against beerus and weis who are Over Powered as hell later on in the game. Best ultimate is finial Big Bang kamehameha which is unlocked in a high rank parallel quest. I personally haven't unlocked it but I've used it. My set up which is destroying enemies is final flash and super Saiyan 2. Having this combo let's me spam super after super. Beat escape move I found is super jump back. Using it will let you escape most situations when you're getting tag teamed by enemies. Best place to farm depends on your skill. The higher the parallel quest and your score they're Zeni and xp you get, I use super Saiyan god goku as I can beat him in 5 minutes and get a decent score on him along with 17k xp each time.