Elemental arrows don't do much damage until the debuff sticks (there's a bar that fills with each shot). The basic impact arrows you have are weak and when you get a better now you'll get hard point arrows which are heavy arrows and do more damage, you'll also get th ability to fire 3 arrows at once making you do even more damage.
The key to fighting machines is using the right debuff and taking down their parts. Some enemies have parts that explode when removed doing massive damage at once, while others have parts that either expose them or take away their key abilities.
