Sawtooth is the first serious opponent you will face in Horizon: Zero Dawn. Killing him is required to progress in the story. Classic ranged assault is pointless, so adapt to the advice given to you by the game - this will make things much easier.
First of all, scan the machine (R3 button) and then display the path (R1) through which Sawtooth will be moving. Move towards that path and hide in the tall grass. Now use the Tripcaster that you bought earlier and set the trap in such a way that the trap will be on the creature's path. You can wait patiently for the target to get there or you can speed up the process by throwing a rock.
When the target touches the trap, for a few seconds he will become vulnerable to your attacks. Quickly pull out fire arrows from your inventory and shoot them at all the pieces marked yellow on the armor of the machine. The creature will start to slowly lose energy and after a spectacular explosion he will permanently lose 2/3 of his health points.
Further tactics depend on whether the Sawtooth notices you or not. If you manage to remain hidden, set another trap and then finish the target by shooting at him. Otherwise, simply keep shooting fire arrows at him until he loses all health points. Shoot him in the eyes and when he jumps, dodge (circle button). A well executed dodge will help you avoid taking any damage.