1. POISON LOL
2. That one form-specific attack of his that deals like a million damage to everyone
3. The fact that I have no wind nukes (I assume that's his weakness at least)
It's also a little hard to distinguish which form I am just from looks sometimes. I didn't even know I was fighting his cold form until he threw Icicle Coffin at me.
1.Your big tanks should have equipment on the make them at least mostly protected against all ailments. ESPECIALLY Tenshi, since her self-buff has bad side effects, you want her to have a Ribbon or Shuttle Body on her, no questions asked.
Useful items for buffing resistances are Stars of Elendil (Or the slightly weaker versions, Phial of Galadriel), Ribbon, Shuttle Body. You'll get some more cool stuff for this in 19F chests.
2.At least two tanks with DEF/MND buffs should ALWAYS be out against Rinnosuke. Tenshi and 16F girl should do, and you might want 3 (Tenshi-Ran-16F should work, equip Ran with some DEF/MND/Resistance boosting gear and make sure her buffs stay up).
With 2~3 buffed tanks out you shouldn't have to worry about game-overing at all, just keep up the buffs and bring in Sanae/Reimu for healing when needed.
The 4th character should be whoever is ideal for damaging the current form, and use Fire Form to focus/heal/etc. Don't take out Fire Form until it's the only one left, as it doesn't have dangerous attacks or any kind of status effects. And take out Spirit easily with a Master Spark, it can cause DTH.
As for identifying the forms, the color will correspond with the ones associated to the element of course; Fire will be red, Wind will be yellow, Mystic is purple, etc. Cold and Spirit are the only ones you could mix up, just keep in mind Spirit looks a lot more white and magic-y.