You can often find these in Shrines, the harder the shrine, the more likely you are to get better Ancient Weapons. Remember that Guardians are most vulnerable to weapons of their time - meaning Ancient Weapons. Since Guardians have a ton of health - even the Decayed Guardians have 500 HP - you'll need durable and powerful weapons to do the job. However, doing this you'll need to make sure your shot counts - as you'll likely be out of stamina and horseless when you land.
If you want to be fancy, you can look for ways to get up in the air - like updrafts caused from fire - or jumping off your horse. The same goes for Guardian Skywatchers - fire explosive arrows at their propellers, and they'll fall from the sky making them an east target. While stunned, you can attack the legs of a Guardian to hack them off - and if you destroy all legs they'll be unable to move. Immobilizing is risky but can pay off in a huge way. You can combat this in two ways - immobilizing them, or slowing down time. Note that while immobile Guardians are easier to hit, regular Guardian Stalkers and Guardian Skywatchers tend to move around, making things harder. When you have stronger weapons and armor, it can be more advantageous to stun them with a powerful bow shot to the face, and then move in to attack with melee weapons until they start to re-orient, then move back for another bow shot.