Ah . My team Ryen Trey Mirel/Elly ( depends on boss element ) Chiyo/ryoko farming boss in Challenge floor make all characters HP lower as much as it can so Trey will take DMG instead ( Warning : It's risk you need equipment with Taunt for enemies not aim ally at start battle ) also you can read walkthrough about challenge floor.
And if you talking about Challenge floor raid you should raid Water Dungeon first for equipment Fire resistant and then go farm Fire Dungeon and once you got Box of Ash used it immediately when start boss fight ( Warning Read boss dialogue probably that they cast spell or just fake just at beginning ) after that go farm Boss Earth dungeon for me Great spirit Kiki is most easy hope it did some help you raid.