This may have spoilers of a hidden unlockable. I know I'm one of the FEW people that played the game for both the story and gameplay. But there's a challenge run I have been doing with many teams. It's where you beat the bonus stage with low minimum BP grind. Items that raises stats and zeals are banned. Tarachi-Tamuhito is also banned. It gives a character resurrection. Putting all of your BP into attack is banned too. Defense and Magic Defense has to be raised along with attack. When a character gets the Earth God's Armor item, that's when I stop putting BP into their Magic Defense.

Characters that were not used in the story stages were used to get the items from training mode. This is to prevent the team I planned on using in the bonus stage from getting BP from there. The only training stages that needs to be played is:

Glory Hounds: Soldier's Tupai
Return of the Uninvited Guests: Kosouga (If I'm using Aruruu)
Warriors - Cont. Senjin Kyuu, Goushin Chokutou, Headband of Courage (Dorry & Kurou)
Warrior's Feast: Satokagura Flute

Karula can get Hookasen from Warrior's - Cont. which gives enemies daze, but I don't recommend her using that. Bajin is better.

The stage I used to grind BP is Primordial Dream - Part 2. This is the best stage to grind BP because it lets you use 11 characters. It's slower than just having Oboro one shot a boss but for this challenge run, this stage is way better. There are 11 mandatory enemies to kill and killing an enemy gives you 5 BP. This stage was played at least 12 times to beat pattern 1 in the bonus stage. Then 15 times to beat pattern 2. Pattern 3 just needs three more. I believe this is the lowest amount of grind required to beat each patterns. After beating pattern 3 for the first time, BP can no longer be given to the characters.

The goal is to reach rank 5 on all three patterns. This can be very tedious because in order to increase the rank you have to beat a pattern five times. The teams that i finished this challenge with so far is:

Male Team
Hakuoro, Dorry, Guraa, Oboro, Benawi, Kurou, Haku, Ukon, Ougi, Kiwru, Mikazuchi

Female Team
Eruruu, Aruruu, Karula, Urutori, Kamyu, Touka, Kuon, Uruuru, Nosuri, Atuy, Munechika
I felt Atuy was really broken in this team.

In Progress
Original Team
Hakuoro, Dorry, Guraa, Oboro, Aruruu, Benawi, Kurou, Kamyu, Touka, Genjimaru

Mask of Truth
Haku, Ukon, Uruuru, Nosuri, Ougi, Nekone, Kiwru, Atuy, Anju, Muenchika, Mikazuchi
This is very difficult team because they don't have AOE Co-op Chain attacks. I was able to beat pattern 2 with them, which is the hardest part of this challenge run. It took me a week to figure out how to beat the stage with them.

https://youtu.be/Ygh7I6c7xz0

This thread was not made to get other people to try this challenge run. I don't recommend other people doing it because how hard it is. I doubt anyone has the time and energy for it. I'm only explaining what I have been up to.