Load the most recent gamesave before the fight with Letho and create BOMBS. Make about a dozen Grapeshot or similar bombs. Now when you fight Letho - just keep running away until his Quen is momentarily down and then chuck the bombs at him one after the other by quickly non-stop clicking the R button - this ends the fight in less than a minute!

Here's how I did it:

Backed him into a wall / corner
Parried and counter with strong blows.

If he Aard's you make sure to run sideways cuz he will usually follow up with a dagger or bomb.

Even better, run around trying to dodge his attacks till his Quen is down, then use Yrden and lure him to run into it, he'll be stunned, run behind him and hack his butt off. Repeat till he's down. Almost all fights in this game can be made supremely easy with the right use of Yrden.

I just started the game so I don't know if you're going to fight Letho a second time, but I beat him in Act 1. I died against him about 10 times but eventually figured his pattern. Wish I had bombs during the time so the fight would've gotten a lot quicker.

I just spammed Aard from a distance, then did a leaping/lunge strike towards Letho, then strong attack followed by a fast attack and roll away and repeat. If he has Quen sign do the same but roll away right after the your leaping attack and repeat untill his Quen shield is gone.

There's a 1 second delay before Geralt fires Aard so I would use Aard while Letho is in attacking animation or right after you dodge his signs or bombs.

just Aard + strong attack spam him to death