Better to do it solo anyway, so you can run from the fire to the boss in less than 30 seconds without fighting anything.
If you REALLY need humanity lay down your sign and wait to get summoned. After you beat them, use that humanity to turn human. You'll also get some souls to help you level up, but a few levels is unlikely to change the outcome of any boss fight unless its hitting an important break point.
Lol I was just fighting them yesterday with my lv1 character, got really sick of dying. smough was pretty easy to kill with lightning +4 club after few hours of practice but giant ornstein gave me trouble so I just ended up makeing a +14 light crossbow and finished him off that way.
"If you want everything you end up with nothing." -Lulu(FFX)