how to get diamonds

Does anyone know how I can get more diamonds? I've already done all the missions, played all the games and completed all the events but I still haven't gotten the complete outfit for the permanent banner, there are 4 pieces left and there are only 9 days until the banner is removed. I don't know what else to do, I'm collecting the chest that I have left to collect, which gives you 5 stars, but for me to get enough diamonds for resonance I would have to collect more than 50 chests and there aren't even all the chests that I can collect. If anyone has any hidden tips I would greatly appreciate it!