This chapter contains a complete walkthrough for the mission The Terror of the Sun. Here you will find advice that will help you to complete this quest in the best way possible.
The quest will start during the long cutscene at the end of the mission Deep Secrets of Earth. After it you can speak to Helis.
Aloy will be thrown into the arena where she will be attacked by a corrupted Behemoth. Dodge and observe the current position of the machine. There are four pillars on the arena. Your must hide behind them in such way that the Behemoth will hit them during the charge.
In order to continue the mission you must trigger two such collisions. Another cutscene will start and Aloy will obtain her equipment. Now you must only finish the machine with burning arrows. You can also force two more collisions with the pillars. This will slow down the machine and give you more time to set it on fire.
Once the fight is over, another cutscene will start. Then you can speak with Sylen. The dialogue will end the mission. A new one will appear in the journal - The Heart of the Nora.