Assuming there is a viable plan to disable all opposition Harry can just state this fact in parseltongue and buy time with that. The dark side of Harry surely can come up with one along the lines of partial transfiguration threads of say Iridium ("Beneath the moonlight glints a tiny fragment of silver"..."). Enough approaches that seem conceivable have already been posted.

Actually he has to state this before executing it because that reduces the risk of the intervention as required per the vow. On the other hand he cannot protect earth when dead so he actually has to choose fighting over dying.

Thus his first sentence migt be something like:

"I have a meansss to disssable all opposssition and cause your retreat or death. I mussst use it to protect the world if you do not reduce the risk of my death."