You can't do that because Rinnosuke's initial form has Rasetsu Fist. Even if you ignore its existence, FIR form has Hell Pyre (or whatever the English patch translated it to), which has a DEF factor of 20 and thus requires Tenshi to have 26000 DEF (after buffs) to take 0 damage from.
What you can do is to level until Tenshi can take no damage from any of Rinnosuke's elemental forms except Hell Pyre (which doesn't do much damage and can be reduced with FIR affinity), Get Meiling to tank you through initial form, then sic on Tenshi. Also, kill Rinnosuke ASAP when he's in final form.
I haven't beaten RInnosuke yet, but I just came up with a plan that will probably work given a few attempts.
Tenshi with State of Enlightenment takes 0 damage from everything except a few things like Rasetsu Fist. So once 3 people have died, you can have Tenshi stall the game indefinitely with focus/SoE spam. This allows:
1) Everyone to regain HP/SP for free
2) Ran to give everyone 100% DEF/MND (and also 100% ATK/MAG if you have the patience)
3) Aya/Sakuya to give most character close to 100% speed
4) All nukes to get 100% ATK/MAG or close
Unfortunately, this will probably drain all of Tenshi's TP, so you will be left with three slots. This should be ok as long as enough nukes survive to the final phase. Also, make sure none of the indispensable characters are among the 3 to die (Ran, speedsters, Reimu, etc.)
When Rinnosuke is on his last elemental form (hopefully not Fire, but even if so, just heal Tenshi every once in a while), stall again, and buff the remaining nukes again. Then just damage race the last form, which shouldn't be too hard as long as you kill him before Military Rule.
You can't predict which 3 characters will be sacrificed, but ideally it will be characters who have performed their functions (i.e.: Suwako after nuking away the WND form, etc)
EDIT: Heaven's Lightning may also damage Tenshi a bit, depending on your DEF/MND % at the time. Make sure to have someone heal Tenshi every once in a while.
NEW EDIT: Magic Drain on Tenshi = RAGEEEEEEEEEEE