No. 6 - Nylocas Vasilias
This boss fight, the third of six in the raid, is the most tedious in Theatre of Blood due to the waves and waves of Nylocas arachnids that spawn in, and if you get lazy, it could easily result in a death due to the arachnids doing too much damage to the pillars.
You simply need to attack each arachnid in its corresponding combat style when it spawns. It is easiest to focus on one style at a time, and do so by going after the arachnids directly targeting the pillars.
The one thing to be careful of here is not to click on the wrong arachnid, because that one will then become immune to all damage.
Once all of the little arachnids are taken care of, you finally get to the boss itself. The boss operates in a similar manner. Simply protect against the style with which it is attacking, and attack by using the same style.