OK so what space marine ranged weapon is more likely to kill a necron(by which i mean have it send back too the tomb) and i mean fluff wise of course

and are the high lords of Terra immortal

Well lets see. Fluff wise, I recon a lascannon is a more fitting way t send the bugger back to the tomb, as it vaporises its opponents, whether a missile launcher just explodes. Although a plasma cannon (fluff wise, not rule wise) would also seem like a decent choice.

About the High Lords of Terra however, no they are not immortal.

melta or mulit melta both fluff wise and rules wise.
and no the high lords are human

ok so do they elect new high lords?

I suppose they would most likely be made up of the most influential aristocrats of pivotal Imperial worlds and powerful generals within the Guard and other organisations.