First and foremost, Quelaag is immune to fire, so don't use Pyromancies or any fire-enchanted weapons in this fight. Quelaag will use attacks with both her human half and her spider half. If you defeated Maneater Mildred earlier and are alive, you can summon her right in front of the fog gate leading to this boss. This boss has many attacks, both physical and fire-based, so it helps to have something like the Black Knight Shield or Dragon Crest Shield. When you're close to her, she will swing her sword in a horizontal line, from left to right. She also has another sword attack, this time it's a stab straight ahead, but it's pretty easy to tell when she's going to use it. For the spider portion of her body, her most used attacks are when she spews lava from the mouth. First is a longer stream of lava, and goes back and forth. Another attack is when she just drops a gob of lava on the ground, right in front of her. Yet another attack is when she jumps towards you and drops a gob of lava right in front of her. The best times to attack are when she does the long spewing attack, but you need to watch out for her aura attack, which usually happens if you get greedy and keep wailing on her. You could probably get one hit off on her when she does the lava gob spit. Keep to Quelaag's left side and most of her sword swipes will miss you, so you can get some attacks in on her from those whiffs.