[JMS] Kanna vs. Some Bosses (And Fourth Job stuff)

That's not very hard, considering Demon Slayer's link skill is 10% boss damage and Kanna's is +10% damage - unless you mean excluding her link skill.

Excluding her skills, so you need another 10% on top of Demon Slayer's, and 30 skill points invested in it. Shoujou Kashu also benefits Shisen Gyouha more because it attacks faster.

Actually, I just re-evaluated it, placing Shugyoku no Juin as priority cast whenever you have 15 red orbs; i.e. after buffing
IF orbs > 15, use Shugyoku no Juin if you have the spirit
ELSE use Haja Rengekifu if you have the spirit (you'll technically only ever run out of spirit by buffing)
ELSE use Shisen Gyouha if you don't have spirit

This assumes Shoujou Kashu can generate red orbs as well.

You need 50% Boss/Total damage to have Haja Rengekifu beat Shisen Gyouha, and if you have Speed Infusion (Fastest (2) speed cap), you need 350% Boss/Total for Haja Rengekifu to win, and I'd assume at this point, you want the superior hits/s from Shisen Gyouha anyways.

Without Shugyoku no Juin, you only need 20% without SI and 140% with SI, since a significant portion of the damage boost by SI is the generation of red orbs.

Shugyoku no Juin is a gigantic damage dealer, since it can hit up to 15 times, and it is very fast. Much like ME for Shadowers, it contributes a lot of damage, and since the orbs don't last long, you should try to time it so that whenever 15 are on the field to explode them.