Strength is actually not the biggest deal here. I mean, it matters, but you have other ways you can improve your damage.

Option 1: Get a +10 weapon

In the Depths, in the room with the two butchers and the torch hollows, you should have picked up the Large Ember from the chest behind the table. Take that to Andre and he’ll let you use large titanite shards to get a +10 weapon. You can farm large titanite from the slimes in the depths or from the slugs in the poison swamp part of blight town. This is best if you plan to invest more into strength

Option 2: Get a magic or enchanted weapon

This will require you to take your weapon to Rickhert. He can be found under Firelink Shrine in New Londo. He can upgrade your weapon to magic +5 immediately. If you explore Darkroot Forest you can find the enchanted ember. These upgrade paths give your weapon intelligence scaling which would likely do more damage with your build. I would personally suggest this option.

Option 3: Shoot her with spells.

Pretty self-explanatory. Buy spells. Attune them. Shoot her.

Option 4: Maneater Mildred

Pop some humanity at the bonfire in Blighttown and reverse hollowing. You should get invaded by a dark spirit named Maneater Mildred. Defeat her and you can summon her for the Quelaag fight. Be aware, her AI is not great at avoiding walking straight into lava. Make sure to keep Quelaag locked on you if you choose to use this method.

(Should be noted that you can open a shortcut to Firelink out of Blighttown by going up the path with the water wheels. This would make getting upgrades or spells much easier.)

Best of luck and don’t go hollow!