He says he comes up with it after losing to Aokiji, but he first uses it and mentions it against CP9's Blueno

The first one Luffy defeated was the marines who took Roronoa Zorro to be executed. He went on to his adventure with Zorro and met Nami and they defeated Arlong, the sharkman who conquered Nami's hometown. The most remarkable opponents he defeated were Crocodile Gecko Moria and Eneru. They were very powerful that they almost killed Luffy but Luffy succeeded.

The major (and minor ones worth mentioning) characters he has defeated;

- Helmeppo,

- Alvida,

- Buggy the Clown,

- Captain Kuro,

- Arlong,

- Don Krieg,

- Wapol,

- Mr. 3,

- Sir Crocodile,

- Bellamy the Hyena,

- Enel,

- Foxxy the Silver Fox,

- Blueno,

- Rob Lucci,

- Oars,

- Gecko Moria,

- Monkey D. Garp,

- Hody Jones.