This boss is just a big fake he tries to act like a rat he is not a rat stop trying to be a rat

Using the shield method, my last swing killed him just as he began his puking animation. The goo covered the floor, but didn't effect my armor (I suppose because he was dead?). Might've been a one-time glitch, might be worth testing. I was using Penal Mask, Smelter Demon Armor, Smelter Demon Legs, and Leather Gloves, with a Greatsword +9 and Tower Shield +8.

The poor thing... When he reaches 20% I just wanna call a veterinary.... somebody put him out of his misery ( ×_×)

Killing the small rats is doable with just Soul Arrow and Homing Soulmass. Cast a Soul Mass, then kill the middle rat, snipe the far left or far right rat, let the other two get closer and shoot the remaining one with a Soul Arrow.

Important: See a way to kill all or almost all rats before the boss comes down and roll the head blows.

for miracle users i reccomend use soul appease on little rats just come near and before they come start casting

Vengarl's armor +3 and a +2 demon's great hammer were enough for me to kill it easily once I smashed the little rats, I healed after he used his swipes and I ran behind him

Played with a Great Sword just stood right under him and Killed him without any efforts.

The Santier's Spear you find in the area comes quite handy for the rats in the beginning, that and pyromancy make quick work of this guy