The highest point in the Widow's Clutch is located at [66, 56] on top of a series of pillars with Cinderweb Spinners sitting atop them. When a Spinner is engaged in combat, it will cast Entangling Net, which will drag the player to the spider. You simply have to keep engaging (and killing) the Spinners until you are at the very top of the highest pillar.
Note that if you die during the higher stages of this achievement, you will not be able to retrieve your body.
[Veteran of the Molten Front].
Patch 4.2.0 (2011-06-28): Added.