I'm having trouble killing Letho near the end of the first chapter. How do I do it?

The following is somewhat of a spoiler, to avoid it being shown in the question excerpt, I'm filling the space with useless boilerplate.

I'm near the end of the first chapter and I'm now facing Letho, the kingslayer and witcher. And I'm getting my ass kicked thoroughly and completely. I haven't even been able to hurt Letho in the short time I'm staying alive.

He shields himself with Quen and just blocks every single attack I make. Then he blasts me with Aard, throws a poison grenade at me, or incinerates me with Igni. I really haven't been able to make any dent into his health bar, he just blocks every attack. And I can't keep up Quen as fast as he damages me.

Any tips on how to kill that bastard? I'm not kidding when I say that I see no way of surviving that fight at the moment.

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.

The fight with Letho in Chapter 1 is challenging any way you slice it, but there are a few tips I can give to at least beat him on Normal difficulty. First off, keep your distance from Letho and dodge his beefed up Igni and Aard signs as well as the various bombs he tosses at you. He will frequently use his Quen sign to shield himself, and it is no use attacking him while it is up. Thankfully the Yrden sign works extremely well; while keeping your distance, drop a Yrden in the center of the fighting area. It will usually take 2 Yrden to break through Letho's Quen and after the second Yrden, Letho will be open to attack. Quickly move in and go all out with heavy strikes (you should at least get in about 4-5 hits before your Yrden wears off).

Once Letho is free, you will probably want to back off. Letho will A) cast Quen, in which case you'll want to simply repeat the previous strategy, B) cast Aard, which does a moderate amount of damage or C) attempt to attack with his sword, which can be blocked with a follow up quick strike or two for a small bit of extra damage. Sometimes Letho will shrug off Yrden if he is unshielded by his Quen. Throwing daggers (especially with 2 points in Dagger Throwing) come in handy for most fights in general, this one is no exception. Grapeshot bombs and traps can also be effective to get some extra damage in after your Yrden wears off.