I'd say that it was difficult for he cast quen constantly, you would have to either run and evade till it wears down and/or hit it with bombs or daggers, don't stay in one place for long so you can evade his bombs easily

I found him very easy just using yarden and back stabbing him

others have said that just railing him with throwing knives works well

There's a lot of tactics for him, you just need to know what NOT to do
Do NOT:
- attempt to spam sword strikes on him. You'll get Aarded away and most likely hit with a bomb
- hit him while he has Quen on, there's no point
- stay in one spot longer than one second while avoiding him, you'll get Igni'd or bombed
- stay anywhere near him without CCing him in any way, you'll get Aarded away
Now, ways to kill him:
- spam daggers and/or bombs, don't get anywhere near
- Aard + 3 or 4 (can't remember) sword strikes, repeat, run away when out of vigor
- Yrden + 2-3 strong strikes to his back, repeat, run away when out of vigor
- when he has Quen on, either run around waiting for it to drop, or keep knocking him down with Aard until it wears off
- keep Quen on yourself and counter his attacks, run away when out of vigor

I learned two things that did the trick with him:

1. realizing that I needed to move constantly, so that he would almost always miss me with Signs; and
2. figuring out the optimal distance I needed to be - close enough that he would use sword strikes rather than throw bombs, and close enough for me to immediately hit him when I used Aard, but far enough away that I could easily dodge his sword attacks.

Once you settle on an approach like that, it's pretty easy - but it took me a handful of frustrating deaths before I found the right approach.

I kept dying until I started using Aard to stun him and then attacking fast, then rolling away and using Aard again.

Beat him first try with almost a full life bar doing that.

This is my experience from hard difficulty.
I killed him very easily-with grapeshot bombs. You will need about 5-10 depending on your alchemy skills.
You can use daggers as well. They do about as much damage but are thrown slowly.
Conventional methods against him just don't work.

MINOR SPOILER ALERT, THIS PARAGRAPH ONLY: you don't actually get Letho's health to zero before the fight ends, more like half way down. So if you're getting him down near half way, you're almost there.

The most important discovery for me was that I can only hit him twice with my sword before he's ready to block. If you attack and he blocks, he counter strikes for like 50. Sometimes I can get three strikes in after Aard. But he's ready on the defense pretty fast, so you have to take your two or three strikes, then dodge or hit him with Aard.

Also, unless you have level 2 parry, you take a ton of damage when he hits you even if you block.

However, if it keeps giving you fits, go to the game options and just set if for easy. ON easy, you can rock him with nothing but your sword.

spam dancing star on the fat bastid when his quen dissipates.