The Giant Spider is the boss of Chapter 6. It will appear on the heart of Magna's body after destroying all its four "lieutenants".
- Arachnid's Claws (heavy-damage melee attack against all targets, takes two turns to execute)
- Pierce (melee attack dealing heavy damage)
- Paralyzing Venom (able to paralyze a target)
- Antivenom (cure all negative effects on itself)
- Frenzy (counterattack after being interrupted, increases all enemies' speed)
- Sting (quick melee attack with a chance of adding a slow effect to the target)
- Pounce (quick melee attack dealing moderate damage)
Boss Fight Strategy:
These strategy tips are taken from examples in the following YouTube video: http://www.youtube.com/watch?v=AECS7iVbV9Q.
Switch out characters, with potions where needed. Concentrate on one soldier spider at a time with melee (Golem is good for the soldiers), and when they're eliminated, take on the boss.
Norah and lull potions can be useful to limit the number of attacks you suffer, although the Giant Spider boss will use the Antivenom ability to cure itself of lull, so you will need to reapply relatively often. Norah likewise can often be used to petrify the boss, mitigating damage it can do. Rubella can be used for healing, and with her ability to prevent negative conditions (slow from the soldiers, paralyze from the boss), it might be worth keeping her out often. Aurora can also do a fair bit of healing, too, although you might want to use her magic differently (see below).
The Giant Spider seems highly resistant against physical damage. While Golem deals heavy damage against the soldier spiders, against the boss its average damage was only around 45 (90-ish on a critical hit).
There might be other effective attacks, but Aurora's light spells and Finn's wildfire proved decently effective, each dealing around 90 damage without a critical. Note that the Golem's critical attack in the YouTube video linked above at 16:47, versus the normal damage of a light attack by Aurora at 16:05 and wildfire by Finn, cast at 19:08.