Mmmm depending on the translationg, the technique is called Ultimate Gyojin Karate:
And from all the techniques used by Jimbei, only Vagabond and Demon Brick are considered secret techniques or have that title. So even if it aint the first, it will be the second strongest. My bet is that it is the strongest since it also uses haki. Jimbei also explains the Gyojin Karate uses water in some way... so...
And once again, Jimbei was very injured from his fight, not a lot to be considered an extreme diff, reason I say it was High Diff since he used some of his strongest attacks and and also some nice injuries. It also seems he PU his armament haki
And from all the techniques used by Jimbei, only Vagabond and Demon Brick are considered secret techniques or have that title. So even if it aint the first, it will be the second strongest. My bet is that it is the strongest since it also uses haki. Jimbei also explains the Gyojin Karate uses water in some way... so...
And once again, Jimbei was very injured from his fight, not a lot to be considered an extreme diff, reason I say it was High Diff since he used some of his strongest attacks and and also some nice injuries. It also seems he PU his armament haki