I'm in chapter 1 at the boss fight with Letho. playing on normal difficulty.

Before the fight I drank Swallow, Rook and Golden Oriole, at the beginning of the fight I cast Quen to protect myself.

In the battle, I try to catch him with Yrden and then attack him. But this does not seem to work most of the time. Sometimes he will just walk right through the Yrden trap completely unaffected (due to Quen?), and when he gets caught, almost every time I try to hit him, he will still parry every attack despite being trapped by Yrden, even if I'm hitting him from behind (!?).

Aard and Igni seem to have almost no effect, and just plain attacking him just leads to a blocked hit with him hitting me afterwards.

So I guess I'm missing something significant?

tl;dr: How do you properly attack people who can parry?

When he has Quen on, he will parry all your attack. Yrden is a fickle ally, whether it works or not depends on chance, for some enemies it works sparsely.

Try to only attack when his Quen is down. For one, distract him so that he won't recast Quen so quickly, e.g. by throwing bombs at him. If he manages to get Quen on, try avoiding him until it wears off. You can engage him, but your blows will be parried, opening you to counter attack (normal hit or an Aard to your face) - you might diminish his Quen, making it expire sooner, but I'm not sure if it's worth taking a hit for that.

Btw, to attack people who can parry. There's one sure-fire way of not getting hit, that is to block the counter-attack. When you block, your opponent will stagger a bit, that's when you get a high chance to hit. You can also dodge when you get parried, but there's still a half-half chance that you will get hit by the counter-attack.

Do not cast Quen on yourself. Drink potion that regen vigor and use skill that regen vigor. Use Aard to knockback Letho when he cast Quen. When his Quen's wear off Aard him to stun him and score 2-3 hits. Use Aard again, 2-3 hits, rinse and repeat. Win.

Well, I would suggest casting Quen when you get your ass burnt though. The shit hurts a lot and you'll need Quen to extinguish it.

If you time Aard correctly, he has no chance to cast Quen and do anything else.

Roll to dodge his attacks. When he misses you, hit him ONCE and roll away from him. Rinse and repeat. Obviously this has to be done when Letho's Quen is down, so keep away from him when his Quen is up.

Keep an eye on your own Quen and re-cast it when it goes down. When there's some distance between you and Letho look for chances to hit him with bombs or knifes, but be careful because he also throws bombs at you, so be prepared to roll at all times, even if there's some distance between him and you.

This seems to have been what I was missing. Thank for all the suggestions, I'll try them now.

edit: Yep, now he was pretty much a piece of cake. Before, I was always rolling away from the counter attacks instead of parrying those. Now I feel ashamed for all those failed tries, haha.

The easiest time I have had with him is when I went into alchemy to get 2/2 gathering and had 100% increased bomb damage, and about 100 bombs in my inventory. Grapeshot barrage to the face with mixes of dancing star, and zerrikanian sun while rolling side to side. He never swung at me once. This was on insane difficulty.

It seems for the most part the game is balanced around melee combat, if you can keep everything at range there is a lot less peril for you, but it takes some fun out of it as well.

He's pretty susceptible to daggers as well, if you picked up that trait.

Or, if you're very very patient, you can walk in circles around the central terrain feature. Wait in front of a corner, toss an Igni as he's about to charge and head towards the next corner. Repeat. Very very boring, but it's low-risk and requires essentially no skill.

(Oh, my bad. Act 1, not 3. Ignore.)

The first time through, I used Igni, whatever bombs I had on me, and a lot of dodging. I actually "won" the fight on the first try. It surprised me, because he hit me and then the cut-scene started.

Like someone else alluded, if you have daggers, Letho is one of those opponents you've been saving daggers for. Use them on him until you run out.

^ This. ^

You can risk an extra blow or two if you knock him back on your first blow. These should be good solid hits and worth the risk. As soon as he blocks or looks like he is ready to block your next blow, roll out of there.

Virga is a very effective defensive potion in this fight. It reduces, by a lot, his ability to hurt you.

Isn't this fight with Letho a scripted one anyways? Noone wins or loses no matter what they do?

You have to win, and then he suddenly has a "Now I'm all fired up, you BASTARD!" moment and kicks your ass. If you lose, you die.

Tossing daggers at him is great, it costs you nothing and rips off his Quen. It also interrupts his bomb tossing.

Basically, play keepaway until his Quen falls off, then Igni->Aard and lay it on.