After you speak to Odo and gain access to the Mournweald and East Forest, you can complete the side quest obtained in the town earlier by rescuing the merchant, whose lovely and concerned wife asks you to rescue him. Since letting him die and opportunistically comforting her on the rebound is not an option (heheh) then you might as well pick up some extra XP and loot. This is a tough boss fight with some hilarious dialogue leading in, so make sure you don't skip through it.
For players who choose Katarina, you're going to find a lot of boss fights difficult due to the high abounts of damage sustained from individual attacks. This means a lot of evasive tactics and utilizing vampiric (life-stealing) capabilities on your weapons and jewelry heavily. Unless you have an been focusing heavily on the block attribute stats, you probably dont have a high enough block setting to simply shrug off the damage.
For this boss fight remember to keep moving, avoid the spider's charged up melee attack (you can see it coming ahead of time) and make sure to switch to your short range weapons (which ideally at this point have some life-stealing capability) to feed off of those easy kills to regenerate. Then after evading and killing the babies, switch over and damage Palefang when they're cleared out. Use the rifle for higher criticals.
Your only companion character at this point (unless you're in a multiplayer situation) is going to be Anjali, who will do some decent damage and give you a periodic healing boost (with the right abilities) and draw Palefang's attention at times allowing you to do some rifle damage with specials.
As with many of the boss fights playing as Katarina, patience is a key. On hardcore difficulty a single slip-up which causes you to dake a direct attack could mean instant death unless your health is very high at the moment. Use your recharges and lifestealing effectively. Below is a video walkthrough: