Originally posted by GunsForBucks:

You don't actually "need" to kill a boss after that until you reach the plains.

I'd say you "need" Bonemass more than Elder, to be quite honest. The Swamp Key is not necessary if you carry some Finewood and make a chair in the iron grate locking crypts. By contrast, it's a heck of a lot harder to find Silver without a wishbone; if you're using the Stagbreaker method, you're MUCH more likely to get bogged down in the Mountains where wolves will nom your face off, something you probably do not want in any permadeath run.