This boss is one of the simplest to understand but it is harder to kill him than it may seem.
His two attack patterns are a lunge towards you, where his head cocks back and he suddenly dives. The second attack is him knocking stalactites and sometimes fire Slymes from above.
The lunge can be dodged by leaving a stack of spears in the wall near you and stand on the top spear. As soon as his tail shakes you leap up and point your sword downwards all the spears will break and a stalactite will land on the snakes head. The second attack is easier to dodge by standing about 2 volgarrs away from the left wall and making the small movement either left or right to dodge the slymes or stalactites.
One of the main ways to deal damage to Fluffy is to stand relatively close to the pile of bones and jump into the air and fire a spear at his head.