If you need a few specific ninjas try to focus on them first, otherwise just focus mostly on power and farm frags of ninjas you really want as you go. Ninjas will come as you go but if you fall too far in power, you'll just lose no matter what team you have. I collected DP Hidan after about 7 different events where he came up. Power helps you earn more. Such as being able to finish Matsuri every week twice will give you extra scrolls to pull for ninjas, finishing all of trial breakthrough will let you farm other ninja frags, being able to plunder for extra coupons, etc. Make use of all the rebate events to get the most out of it as well.