It's strange how so many players have such entirely different opinions regarding the difficulty of this fight. To me, it turned into one of the most predictable and simple fights in the game, once I had been given a few minutes to figure out the patterns involved; once you do that, and are at least somewhat familiar with classic action games (or even some platform games), you can usually finish the whole fight without ever taking a hit. Then there are many others who find it insanely difficult, even after trying for over an hour. Since I can't relate, it's hard to figure out what exactly is the problem (without seeing a demonstration first) - not counting any QTE problems due to low FPS or what have you - so it's hard to give any more than just general advise.

Prepare with potions: Mongoose for poison resistance, Swallow for vitality regeneration, plus a third potion of your choice, preferably a second potion that helps your vitality regeneration.

Take at least one point in Feetwork: This is probably the most used skill in the game. No matter your build, you'll be hard-pressed to find anyone that doesn't take at least one point in this talent. It makes dodging those tentacles so much easier.

Find the safe spot: One aspect of this fight that turns it into much less of a challenge, is that you can actually stay in one place most of the time, safely out of range of both its tentacle attack combo (after dismembering a tentacle), and its poison spit. I usually go to the left side, and stand back at a comfortable distance in the corner there. This is the place to retreat to each time you successfully remove a tentacle, as the kayran then goes into a series of attacks and/or spitting poison; each of these attacks are almost directly in front of it, so as long as you're standing at an angle, he can't hit you. Simply wait for things to simmer down, and resume the fight.

Swiftly place your traps: When the kayran is not in the process of flailing its arms around madly (as per the combo series mentioned above), you have to trick the two 'vulnerable' tentacles on each side to slam down on your Yrden trap (and your kayran trap, if you made one). Line up 'vertically' with whatever tentacle you want to trap - whichever tentacle is more 'vertically' in line with Geralt, will perform the slam attack on him. If you're staring straight at it, it's just a matter of rolling out of the way when you see him raising it in preparation for the attack. Once the tentacle has started moving, it has already 'decided' exactly where on the ground it will hit, so all you need to dodge straight left or right to get out of the way. Right after dodging one of these attacks, quickly run back to where the tentacle landed, and place an Yrden trap (or the kayran trap); then stand on top of the Yrden, waiting for its tentacle to be raised again. As soon as you see it being raised, dodge out of the way again. Barring any random oddities, the tentacle will land right on the trap, paralysing it. Simply run up to the pulsating red lump of the tentacle, and start hacking away.

Once you've got this mastered, it's a simple matter of rinse and repeat. Hack of the second tentacle on the same side; after that, you must move over to the other side. This can be done while he's in the middle of his attack combo, immediately after destroying the second tentacle, using clever stop-and-go techniques and dodging to not be hit; but it's obviously safer to wait until after he's calmed down and you can quite calmly saunter on over to the other side. Once you're there, line up with one of the two outer tentacles, luring it to try attacking you - once you've expertly moved out of the way, place your Yrden trap as usual, and do whatever you did back on the previous side. Once the third tentacle has been removed, stand not too far away from the kayran, but definitely not too close either, and await the next stage. It will make a big, wide swing at you, and depending on if you have Difficult QTE's set to on or off, you might have to quickly right-click when it comes close to you, in order to latch on to it (if the setting is switched off, Geralt will do this automatically). Then some turbo-clicking (left mouse button), and then be ready to hit spacebar to jump off at the appropriate time (again, this is not needed if the setting above is turned off).

If you're successful, the final stage begins, and this is even easier than anything else in the fight. As you see the bridge collapsing, you may have noticed it forms a very convenient ramp up to the now-trapped kayran underneath. Start heading for it as soon as you can, and climb up it as fast as possible. There might be the occasional boulder thrown at you, or a tentacle slam, but I've found that if you're fast enough to start making your way up there, the kayran never has the time to react - still, pop a Quen before you start moving, just to be safe.

Voilà! Since the entire fight is so heavily based on finding the right pattern, it can be done without ever taking any damage, so there's no need to worry about your Geralt being under-powered or under-equipped in some way. With some practice, and some general attentiveness and quick reflexes, it shouldn't pose much of a problem even if you are completely unlevelled and are fighting it without any pants on. :P