The Queen's Gambit becomes available after you've completed the Traitor's Bounty quest. Go to Three-Toe Huadiv's location to start the quest.
Speak to Three-Toe Huadiv about clearing the pass. Aloy will have to kill the Rockbreaker herself. Destroy the components on its legs to keep it from digging underground. The weak spot in its back is an easy target, aim for that to deal extra damage. Kill them and clear the pass.
Prepare the Ambush
After the cut-scene with Vanasha, you must now set up an ambush. Set up traps in the area and stay in the tall grass to avoid detection. When the cultists pass through kill them and their machines.
This ambush will end up in a series of battles ending with a corrupted Thunderjaw. Destroy the Thunderjaw. Take out its weapons on its back and around its mouth. Tying it down with a Ropecaster will also help greatly.