So...I'm sure there are enough tips and tricks about this out there, mostly consisting of dodge and hit and aard and bombspam him. Since none of that worked for me, I had to find my own way, which is a bit cheaty, but who cares. On Dark Mode and i believe on Insane Mode even more so that guy is EXTREMELY annoying and difficult. So, because I'm sure other people have screamed and raved at their monitor for that very same reason, here is my suprisingly brilliant solution (I thought it wouldn't work, when I first got the idea XD but surprise surprise...):
When you fall into that whole during "Rose of Remembrance" with Triss, you can access the area, where the fight with Letho will commence. Simply aard the wall behind the greenery, right where later you will fly through the wall. Set up snares everywhere and than cover the area with conflagration traps. Commence with the story like normal. When the big fight finally comes, the area now is rigged with your traps. Make sure you cast quen instantly and run to the other side oof the room. the snares will keep letho frim casting Quen. And soon as he triggers the first inflagration trap, if you placed them right beforehand, the whole room will go up (that's what the Quen is for!!) and letho will have zero hitpoints left. Only thing left to do (because BUG! ^^o) wait out his Quen and dodge the bombs he throws at you, then try and land a hit with your sword...et voila, problem solved. Also levelling up your bomb and trap skill to +100% dmg helps tremendously...^^o

Sounds like a nice trick, if a little cheaty ;)

I found that fight extremely frustrating too. Tried sooooooooooooooo many times, over several playthroughs, but simply had to give up, and play on easy to beat the fecker.

I say...I had to go back to right afte the kayran to reset my perks cause everyone was praysing that damnable bombspamthingy...i failed utterly. That butthead does a quen under the barrage, no matter what bomb, and constantly aards me across the room, then bashes my head in...yeah, VERY frustating. Until I head that grand idea that is ^^ never gonna get stuck on this fight EVER again!!! ^^ *happy desu*

Same old story with the bossfights in the Witcher-series: On the first sight (and on the second too) they seem to be hard or almost impossible. But as soon as you find the king's way(s) it get's easy. The clue: Geralt should be aggressive - and stay in melee. He shouldn't run away. If he attacks immediately Letho has limited resources. As soon as the player gets control after the cutscene Geralt should cast Aard to prevent Letho from using Quen. And then Yrden is his friend:

https://www.youtube.com/watch?v=S1z-RAPuYMc

(DL: dark)

I tried that. Many times. Didn't work. I'd be in the middle of smacking him up, and suddenly I was on the other side of the room feeling dizzy. Quite frankly it didn't feel like a fair fight, because Letho was doing things that was impossible. Like Aard, shield, bombs and a wicked wide sword-swing in a second. I read guides and saw videos (including yours), but it was impossible, after tens upon tens of tries. Tried many different strategies and approaches, and got him down to maybe 50% health a couple of times, but ultimately I'd die. At some point I simply wanted to progress the game instead of banging my head against an 'impossible' fight.

Think this fight was even worse than the Beast in Witcher 1.

You're too low for this fight if you don't survive. Level up some more and finish all side quests and monster contracts.

Even in insane difficulty Letho is so easy to beat you just need to anticipate his attacks and they are pretty much predictable.

There are always the noobs (like me) who are simply unable to get those fights down to a science and go for a more instinctual approach...which clearly doesn't work with Letho XD Also, why doing a long drawn out complicated fight when there is an easier way?

You can also beat the second act's boss with traps like that. And the optional superboss toward the end. Takes a little doing, but traps are absurdly powerful against most everything. Especially snares, since they can be stacked into a giant pile of death and are reusable. Good times.