To get Moon Breathing in Weak Legacy 2, here is the essential process:

• First, you need to acquire the Kamado Clan and the Suki Gooni Clan, which have low chances to obtain (around 0.5% and 0.1%).
• Next, you must get the Sun Breathing style and level it up to mastery 200 because unlocking Moon Breathing requires having Sun Breathing first.
• After reaching the required Sun Breathing mastery and level (around level 300), you then fight the Moon Breathing boss to get the Moon Breathing Soul, which is rare.
• You also need to gather 2.5 million Yen and reach high mastery (200+) and level (200+) for Moon Breathing skills.
• Once all requirements are met, you unlock the Moon Breathing style and can start learning its moves, including powerful techniques like Dark Moon Slash.

Note that obtaining Moon Breathing is very difficult due to rare drops and high requirements, including having Sun Breathing and max mastery on it first.