We know that Z lost his arm to a pirate with a DF power who became a Warlord during the timeskip (one year before the current timeline). Since only Buggy, Law and Weevil became Warlords during this time, the only option possible is Law. Buggy is not strong enough neither had a great bounty (which he should have if he had defeated a former Admiral); Weevil doesn't have a DF power. Law is 26 and was a rookie at the time Z lost his arm, which would justify his 200 million bounty.
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