This Reddit post has the same question, and leads to two videos (of the demo) of players defeating guardians: a moving one, and an immobile one.
Moving guardian : the player turns around the guardian, dodging its attacks, and firing explosive arrows to weaken it.
Then he fires an arrow directly to its eye, which destroys it.
Immobile guardian : the player fires arrows to the guardian eye while it is preparing an attack.
Then, between each guardian fire, the player hits it with his sword, until the guardian is defeated.
The player will eventually dodge the guardian's attacks, as they are one-shot-attacks.
Also, it has been proven that any shield can parry a guardian laser attack back at it to defeat it.
This is the quickest method, though you need good timing.
Immobile guardians only require one parried attack, but mobile guardians require that you parry 3 attacks back at them.
Ancient arrows will also make short work of guardians, purchasable at the Akkala Ancient Tech Lab.
gaming.stackexchange.com/users/107600/V4karian
Guardian Stalkers' legs are vulnerable to attack.
And so are the propellers of the airborne ones according the to loading screen tip.
My preferred method is to shoot the eye to stun it, then attack & destroy a leg.
Keep stunning it and going after the other 5 legs to do quite a bit of damage.
By the time it is legless, you can finish it off easily.
Be sure to grab the parts the destroyed leg drops!
gaming.stackexchange.com/users/9300/CyberSkull
You can deflect the laser back to the Guardian with any shield, the timing does not change regardless of the shield you use.
It takes three deflections for a regular Guardian and 1 deflection for an immobile one.
The easiest method of taking out a guardian is to use Ancient Arrows.
They can be obtained from Robbie at the Akkala Science Lab.
Each Ancient Arrow will one shot a Guardian if you hit it directly in the eye, both moving and immobile.
Otherwise, it will deal a large amount of damage.
gaming.stackexchange.com/users/145420/SGR
For moving guardians, I usually get into the air above them to drop down with my drawn sword.
After that I am trying to destroy the first leg, if not successful, eat a speedboost and sprint for the chosen leg, which usually succeeds.
Now it is just a matter of aiming for the correct areas.
Trying to destroy the next leg while simultaneously hitting the body for general damage.
Each time you destroy a leg, the guardian won't attack and if you are lucky, you are able to make it "roll over" which allows free hits onto the body.
Repeat that until no legs are left and pray to god that you are able to kill it by then.
If that does not work out, I try to parry a shot which should do the killing blow.
gaming.stackexchange.com/users/126214/Marv
With the immobile ones if you sneak up behind them and whack them with a charged attack from a two handed weapon you can usually kill them before they can even lock on to you.
I'm not entirely sure (as I think I have seen them spin right round) but the immobile ones seem to have limited turning range so if you come up behind them they can't attack you at all.
They go a bit crazy and start firing off randomly if you get it right but they shouldn't hit you.
Cutting off the moving ones' legs has already been covered, but the best tip really is get the Ancient Armour (even just one piece) and the Master Sword as these two things make killing them a breeze.
gaming.stackexchange.com/users/184817/adaliabooks
You could use the parry with (any) shield if you're reckless enough, just get the timing down and practice, or, you could get the master sword (located in the lost woods, follow the fire, get the torch,
light that on fire, then follow the embers that go off of you
're torch, and keep following them until you're there.)
And it should take about 3 hits for the ones that can't move.
gaming.stackexchange.com/users/185576/Bagel
For mobile guardians it takes 3 ancient arrows to the body or 1 to the eye.
Alternatively, you can reflect the laser 3 times to kill it.
For immobile ones, it’s the same, it just takes 1 ancient arrow or 1 reflection.
gaming.stackexchange.com/users/233619/Dragonmaster
I have not been able to discover a method of defeating guardians. I have to run away every time. What are some methods to defeating these giant instant death laser creatures?
How can I defeat guardians?
gaming.stackexchange.com
<zelda-breath-of-the-wild>
gaming.stackexchange.com/questions/302014
gaming.stackexchange.com/users/71074/GarrettJ
For moving guardians,  it takes 3 ancient arrows to the body or 1 to the eye.  For immobile guardians, the player fires arrows to the guardian eye while it is preparing an attack. If it is mobile, try shooting an arrow into its eye while dodging its attacks. For the immobile one you can sneak up behind them and hit them or shoot at the eye. Other ways that could work are deflecting the laser back at it, parrying its attacks hitting its legs or shooting the eye to stun it.
Solution for mobile guardians Solution for immobile guardian Other tips