Going into this battle early on can be suicide. Make sure you have the right stats and equipment before even attempting this. This is an optional boss so you don't actually need to beat this boss to beat the game.
Having a shield that blocks 100% of physical damage helps quite a bit. I use the Royal Kite Shield. Equip your strongest weapon and head into battle.
Two gargoyles will spawn first. When you get one gargoyle to 50% health, another one comes to life. Focus on killing one at a time. If you manage to accidentally get another gargoyle to 50% health while the first one is still alive, another one will come to life and you'll have 4 chasing after you.
Avoiding a single gargoyle's attack is not too hard, the challenge arises when you have to keep your eye on multiple ones. You may be able to avoid one gargoyle's attack, but chances are, another gargoyle is preparing to hit you. Use your shield when necessary. Most of their attacks can be blocked with the shield except for when they either breathe fire or electrify you.
Defeating them nets you 'Belfry Gargoyle Soul' and 25,000 souls
P.S. I also added a little bit of invader footage at the beginning for fun :p