Marineford was Luffy's hardest challenge, pushing him physically and emotionally. Luffy almost gave up after Ace's death and ...

Luffy beat Kaido and became a Yonko, earning a 3 billion berry bounty ... which gave him access to Gear 5. All these powers combined eventually led to Luffy defeating Kaido.

Luffy activates Gear 3 and attacks Kaido, who, much to his dismay, gets knocked down.