How can I defeat guardians?

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.

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 legs drop!

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.

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.

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.

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.

For me, I fly from a high place or use Ravali's gale to do a midair arrow strike (I'm not sure what it's called) and shoot either bomb arrows or shock arrows at it. This will stun it and I use either my master sword or guardian sword to destroy its legs to continue to stun it every time one leg is broken. If it wakes back up, stasis it and wack it a few times. Then continue to do what I said and when all the legs are destroyed, there is usually 500 or so hp left so either continue whacking it, or shoot arrows to it's eye. either that, or use stasis to freeze it and shoot an ancient arrow to the eye to automatically kill it.

For me, I never fought the guardians but I do know a way, it is too whack it with a sword that deals a lot of damage, and then when it locks on you with laser, shoot a arrow at its eye. This will disable its attack as long as you shoot it in the eye everytime it tries to shoot you. To get close enough to a guardian, if you have the time freeze+ rune (i'm not sure what its called yet since I don't have it but if you do it is the yellow lock with a +) time freeze it so you can get close without being shot. This strategy works well with immobile guardians. For mobile ones, time freeze them and then they will act like immobile guardians. Warning: Strong guadians won't be frozen for very long so act quick. All guardians can be time freezed when they are about to attack, this is a useful way to fight guardians. Otherwise, just try to avoid them.