I know the feeling. I died about 5 times until I figured what works for me. As soon as the fight starts, I entered my inventory screen and applied some increased damage oil [forgot the name], as I wanted to try the fight without pre-loading the sequence and gulping Swallow etc...just for shits/giggles. Apply your shield. Then roll around the floor within reasonable close proximity. I believe he'll stop tossing bombs if you close the gap. Wait for his shield to drop [he's on a timer just like you] and then jump in for a few hits. Blocking his strikes and counter-striking works. You may not be able to avoid all his sign attacks but should be to avoid most if not all the bombs. That works like a charm. He may try his whirlwind or whatever sword attack but just dodge out of the way and try to land a couple hits on his backside. Rinse repeat.

That's how i beat him on hard (my favorite boss fight btw)

This is the fight where you want to use block. Keep using queen sign whenever possible during the fight. When Letho is under shield as well set a yerden magic trap and keep beating him from behind. Strike with few blows when he looses his shield, then after he counters one of them the chance is he's gonna use aard on you(that's why queen is important) or try to slash your face(use block in that case and try to land few strong blows).

Drink some potions b4 fight(swallow and the one that regenerate your vigor are preferable).

Good luck!

Use swallow, use rook, and use tawny owl.

Yrden, use it. Use it, use it, use it. It's probably the most under used ability that I've seen so far but it makes the fight trivial once you figure him out. Also, always get behind him while he's stunned or trapped. It's an automatic 25% dmg bonus, more if you put points in for it.

As everyone else mentioned, avoid his bombs and signs. Bait him into your sign and then smash away at him.

I didn't use any signs in this fight except for aard. I just rolled around dodging his attacks until his shield wore off. Then I hit him with aard, got a couple of sword hits in, hit him with aard again etc. When I was out of vigor I just backed out and let him redo his shield, then just repeat the whole process.

I defeated him this way and only took like 1-2 hits.

Use rolls.
My strat was this.

Roll around until his Shield goes off.

Constantly rolling will save you from his pushes, bombs and other signs.

Swallow doesn't really help in this fight, because if he catches you, he hits you for so much it is not worth much.

So roll until his shield goes, then use your quick to have Geralt zoom to him, do a few combos, light to heavy.

1-3 hits is all you are going to get on him.

If he starts swinging the sword like a mad man, roll away.

Just a game of whittling his health down.

You only need to get him to 50% health, so don't give up!

This is how I did it. Run/roll in circles around him until he throws a bomb or does aard/igni. You then get a little while before the next cast. During that time I placed a yrden trap on the ground. It doesn't always stick him but about 50% of the time it does. Once he is trapped he is stuck for around 5-10 seconds. If his shield is down you can run behind him and make a good 3 heavy hits on him (maybe more) before he recovers - this deals a ton of damage to him. Roll away immediately after. If his shield was still up don't waste time hitting instead you can plant another yrden trap near him so he activates it almost immediately when he recovers.

I had used potions, oils, etc and couldn't do this. But thanks to the advice in this thread it became trivial.

Open with a strong attack (stops him getting Quen up) then just spam knives. The fight is basically over at that point. :)