Well, first of all don't bully people No, just kidding. I don't have much power myself (around 60.000) so don't expect too much from me . It's true that ninja assist and mount Myboku (how tf do you spell that?) give a lot of power, but if it's getting expensive I would say you should focus on group skills. Those gave me a lot of power (around 5-6k). I heard that summons give you a lot of power too, so try to get some unbound runes and upgrade them. Oh, and maxing out the battle armor (with the cloth) gave me about 1.5k power. I dont know much about the secret scroll, so I think someone else has to help you with that.

I think this is everything I could tell you and yeah, good luck.

EDIT: About the coupons, I think you should spent them in events like treausre of the Sage