It cannot be otherwise:
- I tumble under his armpit, he still hits me with Aar spell.
- His wirlwond attack hits everything in the room also if three meters far from him. Thus the hit box is a mess.
- Tumbling around him while he is protected with Quen is pointless, after the third try he anticipates you with Arr and Geralt
is thrown against a wall, stunned and ready to be sliced.
- Trying to change direction is pointless, he is faster.
- Bombs are useless, he is twice faster in casting Quen
- Throwing knives? C'mon
- He only needs two hits to kill Geralt at lvl 10
- He can parry from behind
- Yrden sometimes works, sometimes does't. If it doesn't you are too close, so you're dead.
- The controls don't coope, sometimes they are unresponsive, so I have to push key twice or more, at this point you miss the timing, and, guess what, you're dead.

So what?

It's always the same with the boss fights in TW1-3, OP: They seem to be challenging as long as you don't know how to manage them. But if you find the king's way (or the king's ways) the boss fights get easy enough. They are some kind of riddle.

Letho has unpleasant ranged attacks. So Geralt has to prevent Letho from using them. He should be aggressive, then Letho has limited ressources.

As soon as you get control after the cutscene, Geralt should cast Aard immediately to prevent Letho from casting Quen - and then don't let get him off the hook.

Don't know your difficulty level: If your DL is "normal" then you should be able to control Letho completely with Aard:

https://www.youtube.com/watch?v=juMEEvPQ9Rs

The king's way is "Yrden" - if you prefer short bossfights. If Letho has only 40 -45% of his LPs the fight is over - but not the dialogue. When Geralt says: "You should have surrendered when you could" he should stop fighting and get into safety until the cutscene begins.

Then you finish the fight in less than half a minute, even in dark mode:

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

And what's to do if really nothing helps? Then switch to "easy" for this fight. Letho mutates to a tinker, uncapable to do anything right but talking - long-winded. He's completely helpless, he can't block at all. After 8 - 10 hard strokes the fight is over, then you have to wait again:

https://www.youtube.com/watch?v=G_hLsP9jsU8

The problem is that some attacks cannot be avoided. I can close him into an angle, but I know I can hit him max twice
before he reacts. I can parry a sword slash, although it costs HPs, but his Aar spell is instantaneous. I can have the reflexes of a fly, but Geralt is still too slow. Hard. I can face common fights without particular difficulties.

https://www.youtube.com/watch?v=juMEEvPQ9Rs I tried, sometimes he is immune.

Knives only do damage if his quen is down and not too significant iirc.
Even when his shield is up, dancing star can set him on fire and do damage that way. The same goes for the operator.