Fumirul Loop

Fumirul should be equipped with: Nature's Union, Serenity II, and whatever else you like (I like Sprinting Stance and Surging Heart)
Oshtor's equipment isn't as important, but Surging Heart is also good on him.

Ideally, Fumirul should be high enough level that she has Grateful Heart II.  This is still useful if you only have Grateful Heart I, but since it only has a 70% chance, it's not guaranteed.

Oshtor and Fumirul both need a little bit of zeal already before you can start this; I forget exactly how much, but one turn's worth of actions are probably enough.

Have Oshtor crit on the first hit of Sosei Oubu on her. She'll echo with Inariame, and you need to crit on the first two hits of that but cancel the third. That will summon Shumari. She can attack after that if you want to gain a little more. Then Shumari should heal her, and she should echo again; crit on the first two hits but cancel the third. That will put her into overzeal, which is where the abuse starts.

The loop consists of:
1. Fumirul uses the first two hits of Hyakkamitsu on Oshtor to make him go next
2. Fumirul gets in melee range and uses her final strike
3. Oshtor moves close to her and crits on the first hit of Sosei Oubu
4. Fumirul will respond with Inariame, and getting crits on the first two hits of that but canceling the third will put her in overzeal again
5. Repeat

For optimal usage, position Oshtor so that he can participate in a co-op chain in order to prevent the enemy from using any reaction abilities. Critting on all of those abilities is important, otherwise Fumirul won't get enough zeal.

This will easily kill almost anything in the game. If you're on a stage with multiple enemies, you just have to make sure the two of them can move from one enemy to another without the loop getting interrupted (which is why those Surging Hearts are useful), and there are a few enemies with passive abilities that can make their turn order jump up in front of Oshtor.