Stuck on Quelaag

I'm stuck on the boss Quelaag and can’t beat her. The main issue is that I’m not dealing enough damage: with any weapon I tried I'm doing at most is 70 damage per hit.

I’m using a dexterity-based build, so I was mainly using the Uchigatana. Unfortunately, it broke when I got to the end of Blighttown, so I had to switch to the halberd, which is the best replacement weapon I have right now and I have no idea how to proceed.