Finally got through C9 Solo. Very well designed dungeon and I like this trend of fixed teams. It allows gungho to really finely balance the challenge.
I can write up a guide that should be very consistent if needed. I wiped a couple times to randomness and bad boards but once I really started to think about how to use actives it became a lot more manageable.
Here's the strategy I've been attempting to clear it with...
F1 Tamas - puzzle and kill. If an important sub gets bound try to stall while eliminating the tamatron
F2 Predra - use Shroots on one predra, match and take the hit, then use the other Shroots the next turn. It's possible to break def on the dark predra with prong, but not recommended.
F3 Zera - one or two 20.25x should be enough to put them under 50%, then just kill. If you accidentally proc resolve, use Sakuya to get them back under resolve threshold.
F4 Meimei - pop Susanoo for safety and in preparation for the next floor. You can kill with a 49x + light prong, so try to set one up.
F5 Refive - puzzle and kill. 20.25x will get under his damage absorb, and he should be near dead by the time he uses that move anyways.
F6 Beelzebub - One of your Shroots should be up again by now. Pop to get rid of the poison orbs. Pop the other next turn if you fail to kill and he does the full board convert again.
F7 Kali - use Nordis, then puzzle to get into the safe zone. Pop Kanna and whatever else is necessary to secure the kill.