Hi there,
Me and my group created two advanced level guides for spirit combos to beat the game above level 20. They are both long and detailed, mostly meant for those who are stuck at level 9-13 like we were for a while but playing these spirits together may also help newer players to get to higher levels than on what they usually play.
Beating opponents at level 15 for example is pretty easy using the guides below and you may get a kick out of seeing strong opponents suffer and learn new stuff about the game in the meantime 
Have fun!
Thanks for sharing!