Before you begin fighting any beast in Horizon Zero Dawn, it's important you know the machine's weaknesses. This will be accomplished by utilizing the Focus and scanning your hunt. But it's also more than that. You will need to know where to find this information on the beast and how to interpret it.
Stop and Scan
Before you jump in, take a moment to duck into some tall grass or behind cover, and hit R3 to scan with your Focus. Machines will be highlighted in blue, with vulnerable components highlighted in a contrasted bright yellow. The information delivered from this scan is vital. It will tell you not only what the main body of the machine is vulnerable to, but also what will damage the yellow-highlighted components. Generally speaking, armor and weapons are vulnerable to Tear damage, while fuel tanks are vulnerable to Blast or Fire damage. Machine bodies may be vulnerable to Fire, Blast, Freeze, or Shock damage.
Remove Dangerous Weapons
Armed with that knowledge, but before going after a machine's main body, it's useful to remove particularly dangerous weapons. These components will also be highlighted in yellow, and can often be picked up and used against them as a heavy weapon type. So, before they even know it, take aim at these components with a Tear weapon. If you have a Carja or Shadow Sharpshot Bow, you can use the Tearblast arrows to knock off many components in one shot. If you can remove a weapon and grab it, you can usually bring the machine down easily.
Tie the Machine Down
If you have a Ropecaster, you can tie down many of the larger machines in order to have a steadier shot at small components, or even to deal some melee damage. Just be warned that it will break free of its restraints after it sustains a certain amount of damage, and so you'll want to steer clear when that happens.
Use the Elements
If the enemy doesn't drop a heavy weapon and you can't or don't want to use the Ropecaster to keep it steady, exploit its elemental weakness. When you shoot an enemy machine with an elemental weapon, a circle coded to that elemental symbol will appear over its head. You'll have to hit it with that element until the circle fills. If an enemy is resistant to a certain type of elemental damage, the circle won't be able to fill. Once a circle is filled, it will go into a status based on the type of damage. Fire will do constant damage, Shock will stun, Freeze will keep an enemy in place, etc. The machines will also go prone at this point, often exposing parts underneath that are very vulnerable to attack.
The one element that almost all machines are vulnerable to is Corruption. This is only available in a War Bow variant, and the material to craft Corruption arrows can only be found on corrupted enemies. It has an effect similar to Overrides, in that it makes a machine fight for you, but only temporarily.
With these tips for scanning a machine and learning its weakness, there will be no enemies you can't take down in Horizon Zero Dawn. For more on finding the right tools of the trade, check out our Crafting and Resource guide. And be sure to read all of our Horizon Zero Dawn guides.