Tortureweb Dryder bosses are powerful elementalists encountered in the Gate of Pain mission. They resemble Terrorweb Dryders (which can be found roaming nearby) but are also bosses. This means damage from their spells is doubled and scaled up also due to their high level. They are servants of Dhuum sent to aid Abaddon and can be found tormenting the souls of those entrapped in the Realm of Torment. The objective of the mission is to destroy them.
They are always found alone, any nearby foes are not linked to them. As such, it is very prudent to take them out separately.
A good time to attack them is when they are using their Soul Torture skill. It takes a full 15 seconds to cast, and they will not stop casting it for quite a while once they're in combat, giving you time to bash on them a bit.
The safest approach with these creatures is to shut down their spell casting either by dazing them or using other means of skill shutdown such as Shroud of Silence, Blackout and so forth.