Push Letho away with Aard, immediately follow that with strong or fast attacks,approximately 2-3 hits, repeat the process until you run out of Vigor, when that happens, run away. If Letho puts Quen up, set a Yrden infront of you to tear it away and buy you some breathing room. Don't attack Letho up close unless you're absolutely sure you want to, make sure you Parry, and attack only when he isn't, if you can stun him by Parrying, excellent, but only Parry when your just about to be hit by one of his blows. Also, make sure your using the Steel sword, as it proves more effective against humans. You can also set up a few traps quickly if you wish, and ALWAYS dodge to the right just before the battle begins, so Geralt can shift away from Letho's Aard if that's his first move.

Push Letho away with Aard, immediately follow that with strong or fast attacks for approximately 2-3 hits, repeat the process until you run out of Vigor; when that happens, run away. If Letho puts Quen up, set a Yrden in front of you to tear it away and buy you some breathing room. Don't attack Letho up close unless you're absolutely sure you want to, and make sure you parry and attack only when he isn't. If you can stun him by parrying, excellent, but only parry when you're just about to be hit by one of his blows. Also, make sure you're using the Steel sword, as it's more effective against humans. You can also set up a few traps quickly if you wish, and ALWAYS dodge to the right just before the battle begins, so Geralt can shift away from Letho's Aard if that's his first move.

Push him away with Aard and hit him with strong attacks when he's stunned (2-3 hits) and when he's shielded just run around and avoid his attacks. The first time he was too strong so I loaded a former save, did some quest and crafted some stronger items.After that he was quite easy to defeat (also use steel sword NOT silver)

Push Letho away with Aard, immediately follow that with strong or fast attacks,approximately 2-3 hits, repeat the process until you run out of Vigor, when that happens, run away. If Letho puts Quen up, set a Yrden infront of you to tear it away and buy you some breathing room. Don't attack Letho up close unless you're absolutely sure you want to, make sure you Parry, and attack only when he isn't, if you can stun him by Parrying, excellent, but only Parry when your just about to be hit by one of his blows. Also, make sure your using the Steel sword, as it proves more effective against humans. You can also set up a few traps quickly if you wish, and ALWAYS dodge to the right just before the battle begins, so Geralt can shift away from Letho's Aard if that's his first move.