First I would vanish.

Then I would hide, off the grid, while I planned how to take that person out.

Then I would do my very best to remove the threat.

Understood is the fact that the set up will have jelled by then, but before I became a new person, in a different place, I'd make certain that that individual would not be around to dog my heels.

Baring that? Knives work too and bombs are actually a better way to ensure death, as long as you don't get lazy about it. Not one bomb, but five, spread around the whole area they will be in.

*Obviously this is all fiction.