Garp hounded Gold Roger.
Garp and Sengoku were mentioned by Whitebeard as "the few old-timers left around".Garp is really strong. I bet he gets around everywhere as he's needed, including the New World.
But she resisted it an instant later, and without any outside interference (like Momonga's marines, who resisted Hancock's commands after Momonga yelled at them). As long as Hancock do not gets what she wants, she's being resisted, right?
She used to be a empress, so I think she perfecly know how to snap out of it but Luffy is a total ignorant.
I still don't know how he did it.