Well. nothing much. Kirin summon would be best, but I suspect you don't have it yet. Ninjas from Seal Treasures aren't that good. If I were you, Id focus on collecting ninja frags from events. e.g. SoSP Naruto, Kushina RHH. You gonna be surprised, how fast number of your ninjas will increase :)

Here advice from player that started playing almost 4yrs ago. DO NOT ever spend ignots/coupons on Seal Scrolls.