Hello, in that level you are , i would suggest keeping them and spending them always once in a month in lucky board mostly.. Lucky board for low level and power can give you plenty of good items, plus you will redeem something from it, ninja fragments or power items such as charm material packs ( i recommend charm packs from there 100%).. if you like a ninja from the event you can get as much as you can fragments of him.. but clearly for me Lucky board for low power and level is the best event at the moment. Take in mind that you need to be smart and see the best combo that you can do with your coupons, if you are lucky and Fukurokumaru's event is on when lucky board is on, you can get rebate items from spending coupons as well. Try to balance your desires and see if you want a ninja more than power items. at this point you will need to make a good cheap formations and then go for full power.