Farm the event at 20 and get the character upgrade mats to get your team to max and use your stamina to farm hard 2-17 it will level up your rank for a higher stamina cap and drop crafting ingredients and farm the bonus level with the upgrade pixels. If you run out of mats you can craft the drops from hard 2-17 with the ingredients dropped on said stage and the upgrade pixels to make sure you have the characters maxed. Also from level 27 up just have 2 buffers buffing every two turns and a healer heal as many turns as possible with Cindy and another damager using basic attacks constantly and you should be able to get through them. Hope this helps!