For one when you meet Iorveth in the clearing after beating the Arachnid do not go with him and tell him you have something to do. Use this time to prepare and drink potions. Also prepare at least 10 bombs. I used Grapeshot and I had invested in 30% damage for bombs so maybe take a few more if you don't have that upgrade. Looking back the 100% second tier for this upgrade might be worthwhile.

If you are normal person on your first run of the game who can't read the minds of the developers and had prepped for the Arachnid thinking you're good for a bit (Arachnid is a quick fight especially with Arachnid Oil on your Silver Sword, plus your buffs are 10 minutes long) you're not unless you bypass all the Iorveth and Letho dialogue which of course you won't do.

Spending your time in dialogue depletes the timers on sword and potion buffs. So much so by the time you get to the Letho fight you're potion and sword buffs are just about to expire making this fight much harder than it needs to be.

If you buff after defeating the Arachnid and before leaving the clearing with Iorveth you just need to follow "the script" because the Letho fight is a script. A lazy one at that.

One would think Quen would be important, that is what the game has taught us, however don't use it here as with Quen active you will not be able to generate Vigor even if you have the Tawny buff. Do not parry Letho's sword attack as it uses Vigor.

Letho will spam Quen and you can't so make sure to Aard him immediately after his Quen expires and before he gets off another Quen. This one mechanic is the so called riddle of this fight.

• Designers knew your buffs will time out but put in the option to immediately go with Iorveth.
  
  
• It's a sword fight but parrying and blocking are useless and will drain your Vigor.
  
  
• Buffs drain even during dialogue giving you more incentive to skip (spam left click) the dialogue and comeback after the Letho fight via an earlier save to see what the dialogue was.
  
  
• It's a sword fight but you're only chance is to stun lock Letho through Aard.
  
  
• Rolling around like a potato not a badass Witcher.

Defeating Letho:

• Prepare after talking to Iorveth even if you have prepared for the Archnid. Do not go with Iorveth immediately. Tell him you have something to do.
  
  
• Prepare potions:
  
  • Brock: Significantly increase chance of causing critical effects while reducing all resistances. You will need this if you get careless and get hit by Letho's flame and / or poison ranged attacks.
    
  • Tawny Owl: Increases vigor regeneration.
    
  • Swallow: Increases vitality regeneration.If you're low on health play as the rolling potato for a bit. If you're doing this right you should not find yourself with low health.
  
  
• Prepare your Steel sword:
  
  • Whirl Oil: Whirl is a highly universal oil. Applied to a blade, it increases damage dealt to foes, regardless of their type. I would say use Hanged Man's Venom or Brown Oil but I am unsure if Letho is resistant to Poison and / or Bleeding.
  
  
• Ensure that you have enough time on your buffs as discussed above. 5 minutes should be good and if you buff before going with Iorveth and spam left click through Letho's dialogue you will have much more.
  
  
• Circle roll around him, rolling like a potato to avoid fireball and poison. A key factor is to roll within the diameter of your Aard sign effectiveness. If you play the potato too far out you'll never finish the fight as your Aard will be out of range. That's why we have so many signs so you can ignore them. Of course one would think to experiment a bit, drop down a Yrden or two but no.... lazy scripting. Developer's note (interpreted) We gave you all those signs so you can just ignore them.
  
  
• Letho will activate his Quen sign by punching the ground.
  
  
• When his Quen drops nail him with Aard (be in range), then heavy hit, heavy hit, quick hit and then dodge out.
  
  
• There are perceptive delays on the controls (unfortunately and I classify as sometimes unresponsive) so you need to be on your game and don't push your luck. Letho will Aard you if you wait too long to dodge out.
  
  
• Before he gets Quen up again tap your bomb key don't waste time aiming, nail him. In fact spam the bomb key for a quick two. Tip: I bound my alternate weapon key to Left Shift. Dodge out and two pinky finger hits to the LShift key.

Rinse and repeat.

Too easy a fight to be so frustrating for many.

Good Luck.