I feel like Vicar Amelia really shouldn't be people's 4th boss unless you're really over leveled. Instead,
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take a left just before the entrance to the Grand Cathedral and go through the witch area to level/get the rune tool and get a bunch of upgrades for your weapon (you'll be at +6 by the time you are done, probably). When you get back to Amelia she will go down in < 1 minute, if you haven't been losing too many souls