The Flower of Balance, Yin, Yang & Flash provides passive bonus to a character for the non PvP mode which can be helpful in your playthrough or just completing it for the achievement hunting

Bonuses of Each Flower of Balance

Flash = normal state
Flash level 5 Passive bonus


Yang = Shinobi state
Yang level 5 Passive bonus


Yin = Frantic state
Yin level 5 Passive bonus

1. Based on my playthrough, the fastest way is to beat up a boss for level 1 to 4 of any of the flower of Balance in Hard Mode (Normal Mode gives slightly lower the mastery exp)

The levels of the flower of balance increase are based on kill count *[possible the duration in that mode & the hits done in that mode - couldn't really confirm this one]*

Bosses = feeds the highest for level 1 to 4, average (probably lower) at level 4 to 5 (both Normal and Hard Mode)
Mobs = average increment for level 1 to 4, lowest at level 4 to 5 (both Normal and Hard Mode)

Attached are the example's done in Homura Crimson Squad Mission 1-1 Hard Mode (Other schools just pick stages with mobs + a boss or just mobs only to grind)

Example 1: Mission 1-1 of Homura Crimson Squad (Boss Only / Hard Mode), Flash grind (Before)


Example 2: Mission 1-1 of Homura Crimson Squad (Boss Only / Hard Mode), Flash grind (After)


2. Basically the first four levels of any of the flower of balance can be raised very fast by beating up a boss in a particular state (Frantic, Shinobi & Normal)

3. For the final level (level 5) of the flower of balance, it's a little slower to beat up bosses to gain the EXP for the particular flower of balance. So for level 5 onwards pick a stage with additional mobs to help increase it

Bonus Tips
Always grind Frantic Mode (Yin) first because that particular mode cannot change to Shinobi Mode (Yang) unlike Normal Mode (Flash)

Success or Failure of the missions does not kill your grind progress

Chapter 1
Mission 1-01 Basic Training

• Limited numbers of wooden puppets
  
• Suitable for "Rock Hard Lady" Achievement as mentioned here[www.playstationtrophies.org]

Chapter 2
Mission 2-05 The Battle Begins

• A variety of enemies to eliminate
  
• Limited time - 3 Minutes [Timer on top right of screen]

Chapter 3
Mission 3-04 Vs. New Hebijo

• 3 bosses
  
• As long as Miyabi stands, Ryōbi & Ryōna can still be beaten up until they KO
  
• Eliminating Miyabi first = Mission Complete

Chapter 4
Mission 4-02

• Wooden puppets
  
• Infinite enemy spawns as long as the rocks are not shattered as mentioned by user tutarsch23 in this guide (Sorry just noticed this guide as well so mentioning it here as credits)

Chapter 5
Mission 5-01 Fighting for You

• Raven, Archer, Turtle, Mouse
  
• 1 Boss.
  
• The mobs feeds loads of mastery as you rush to the boss
  
• Tested with Ikaruga (in Hard Mode) with 1/4 of Lv. 4 Yang to Lv.5 Yang. Instant lvl 5 Yang upon mission completion

Shinobi Girl's Heart
Mission 6-05 Goodbye Hanzō (Daidōji only) [Hanzō Academy]

Last Battle (Asuka & Homura only) - Requirements: Complete all schools story

Chapter 1
Mission 1-01 Enjoy Life!

• One boss only

Chapter 3
Mission 3-01 Good & Evil

• Mouse, Archer & Raven Shinobi
  
• 1 Boss

Mission 3-02 Only Skin Deep

• Fox shinobi
  
• 1 boss
  
• Tight space for Aerial Rave
  
• Fox shinobi continuous spawn as long as boss stands

Mission 3-05 The Path of Good

• 2 Lion Shinobi (Samurai)
  
• 1 Boss

Chapter 4
Mission 4-01 Secret Bibliophile

• Archer & Turtle Shinobi
  
• Turtle Shinobi will spam guard (in hard mode especially)
  
• 1 Boss

Mission 4-02 Who Wants to Play?

• Raven & Mole Shinobi
  
• 1 Boss
  
• Poison & Undeground Ambush

Mission 4-03 Small World

• Plenty of enemies including Turtle Shinobi
  
• Group them up and hammer them for quick kills

Mission 4-04 Free Spirit

• Bear & Archer Shinobi
  
• 1 boss
  
• Plenty of Bear Shinobi

Mission 4-05 Light in the Dark

• Lion (Samurai) Shinobi and Bear Shinobi
  
• 1 Boss

Chapter 1
Mission 1-01 New Hebijo's Signal

• 1 Boss
  
• The boss can be a hassle to beat due to the small spaces for aerial rave

Mission 1-04 Der Freischutz

• Variety of enemies towards the boss
  
• 1 Boss

Chapter 4
Mission 4-01 Supply and Demand

• Plenty of enemies all the way
  
• Highly recommended due to it's simplicity
  
• 1 Boss

Mission 4-03 The Bond

• Wolves, Moles & Raven Shinobi
  
• Beware their charge attacks that freeze and poisons
  
• 1 Boss

Mission 4-05 The Last Word

• Bears and Wolves Shinobi
  
• Quite a hassle due to these Bear Shinobi like to charge roll (in hard mode)
  
• 1 Boss

Chapter 1
Mission 1-01 Training at Dawn

• 1 boss only,
  
• the boss will prove to be a little hassle but it's quite worth it (in hard mode)

Chapter 2
Mission 2-04 Oil and Water

• Sufficient amount of enemies
  
• Archers and Turtle Shinobi mostly that likes to keep distance
  
• Turtle Shinobi loves to guard
  
• 1 Boss

Mission 2-05 Pulling the Strings

• Sufficient amount of enemies
  
• Raven and Mole Shinobi
  
• Poison & Underground Ambush
  
• 1 Boss

Chapter 5
Mission 5-02 Reason to Fight

• Foxes and Archers
  
• Get to the boss and kill the archers only (they will keep respawning as long as the boss stands)
  
• 1 Boss

Shinobi Girl's Heart
Mission 6-05 Love & Peace (Rin only) [Homura's Crimson Squad]

Last Battle (Asuka & Homura only) - Requirements: Complete all schools story

Hopefully the guide is helpful and have fun. Feel free to drop comments to help improve the guide or enquire on the methods

Other guides written for this game by the community:
"Don't Worry About It" Achievement Guide
Easy EXP with the Hanzo girls.
"Please No More!" Achievement[www.playstationtrophies.org]
"Rock Hard Lady" Achievement[www.playstationtrophies.org]



Grind Asuka (of Hanzo) and Homura (of Homura's Crimson Squad), max all statuses for both of them to enjoy the Grand Finale after completing every school's main story missions