You get twice as much mojo but no exp/cash if you select your opponents in the leader-boards tower. Thats probably how people get more mojo than you. Also there are whales who will drop their kobans on refills, but I don't think there is lot of them.
As a side personal note: You should just give zero fucks about leader-boards and PvP in over-all. Do it if you want additional exp and have all girls dropped from trolls, otherwise do trolls for better cash/h. At least till they maybe someday fix the fucking PvP, by adding seasonal resets and/or level based tiers/brackets.