If by 680k means latest stage of Kodaibana or broken Aqua shadow, you may need at the VERY least 4 rainbows for each team, with dodge and provocation equipped of course.
My team can take on even Niconico 3 stages fine now. (Even with crappy rewards, it's great for testing team set up) and I currently have 7 rainbows in active teams.
What happened?