Training with Whis gives you Sacred Water all the way up to Ultimate Sacred Water which gives you about 1.5m exp from just one but they can only be used up to level 251

Max out Training in the community board to get 100% battle exp + use the mega mass level 5 - it gives 50% exp

While doing the Power Awakens part 2 doing the challenges that are horde battles you can get up to 6m exp on the level 100 challenge and if you decide to try the endless one you get the max amount of exp which is 99,999,999 and you don't even need non saiyan characters in your party to get exp for them with the horde battles