Stratagem (1) State something that is true, but that LV won't believe. Either LV thinks you've broken the Parseltongue curse, or you gain time in the confusion. Him thinking that you've broken the curse gives you a power he knows not that you can bargain/threaten with. Sub-suggestions: "Sometimes we make our own phoenix tears" (when asked why he told his friends to refrain back near the start) ; "The solar system will die in 10 billion years and you will be forever alone" ; "Hey, you know how you forged a time-turned letter? Well, it didn't actually include my code-word for time-turned messages... I wonder if the great Lord Voldemort can predict what will happen now?" (not a lie).

And someone else made the suggestion of making statements that have a true consequent so that you can make up the antecedent along the lines of "If you destroy me now, the sun will die, and the starts blink out one by one. I know not when, but it shall cause you great grief and misery teacher. If you allow me to live, shall keep them alive for as long as I can, remember my vow"