Yeah, attack the guy you're trying to recruit without warning.
Sure, if he can't block your full-power strike, then he's not the guy you want (and better hope you have Diplomatic Immunity because you've just murdered a presumably-pretty-important foreign national in public), but if he can, then you're potentially driving up the price your Queen will need to offer to get him to come along with each failed strike, as well as proving to the country he's currently in (and possibly working with) that he's the real deal and can stop your country's Sword Saint.
That's another reason to try to marry him now, Queen's probably going to be a bit miffed about the result of this encounter, might need sanctuary in a different nation afterwards...