I strongly suggest playing a different difficulty level then, because you'll probably need help for many more fights in the game... Especially since you only need to get Letho to about half health. :/
PS: Use Quen and dodge. Be patient, and don't attack him mindlessly. Wait for him to make a move first, especially if he's using Quen. You should try using Igni or bombs to take down his Quen shield, and if he attacks when it's down, dodge, then attack him. But don't dive into his sword. :p Try getting him from behind. Also, don't spend too much time standing still or attacking him in one spot, he'll probably knock you down with his Aard sign, then throw bombs or Igni at you.
If it makes you feel any better, at least you can play the game. The stupid enhanced edition files are corrupted for me, and I can't install it.. and I can't even play the current version. :\
Post edited April 20, 2012 by Raggit