I would think solo would just never end, regen of 1%/tick when it has blaze spikes up (which it will quite a bit) is probably more damage than any one job can do while keeping shadows up and healing.
But 3-4 should work, have done him on 5 I think at 90.
Edit: I would imagine you would want to have a thf for Hachiryu haidate anyway.
Aren't his spikes dispellable? Dispel is a pretty accurate spell most of the time, even subbed. I'd think the main issue would be killing him before he starts resisting.