"How would you like it if somebody did this to your people?"
That's the best thing I could think of honestly. Or maybe "We're just as sentient as you."
Both highly unlikely to work though. If the advanced alien race about to kill us is willing to do so, they probably come from a culture where they grew up accepting "survival of the fittest" as one of their most important truths. A single sentence from me probably couldn't change their opinions.
I could try saying "Bang.", and speak it out of a gun. Maybe if the alien I spoke with was their leader or at least a general. I think that would give us the best chance honestly.