Save Coupons until you have 20k, then pick up a strong ninja like SoSP Naruto or if you want to Skill Break Kushina (Mad Momma) then you'd need to 4* her then use vials. But I'd recommend SoSP. If possible, spend during a Fukurokumaru Deals when you have enough for SoSP that way you can potentially get 2 good ninjas for the price of 1.