-Run/farm a quest or the daily tanuki quest, keep any 4star+ tanukis. Don't ever use 3star tanukis, it's a complete waste of EXP.
-Go to the conversion center (in the senki tab under sell units).
-Go to bottom and exchange 5 3star tanuki for 1 4star tanuki, you can choose the type you want and you can choose any 3star+ unit to exchange. It just needs to equal 5 luck with 5 characters maximum.
-Level up.
A single 3star tanuki gives only 6k exp while a single 4star tanuki gives you 60k exp, this is without same type bonus. In short, you're basically netting an extra 100% exp and on a single unit, saving you 4 slots.
This method doesn't work for getting 5star tanukis as they require a total of 5 luck with a max of 3 units with only 3star units or higher. You need to merge units to meet the requirement. You can't merge tanukis with tanukis and merging foxes with foxes doesn't increase their luck stat. Instead, don't sell those useless 3 star generic units you get, keep then and merge them to trade in for the 5star tanukis. Now they're a little less useless.
6stars are more difficult to get requiring a single 4star unit with at least 10 luck. For free players, I feel the best way to achieve this is farming those girls from the daily event missions but they're not guaranteed drops and it changes everyday, so it may take several days based on your luck and stamina, but hey, unless you need them for godly evolution, there isn't much else use for them as they're weak and sell for 1 senki point.