Well indeed... Kayran fight is stupidly hard, but Letho is much, much worse. I was level 12 this time, prepared as well as I could with whetsttone, upgraded armour and sword, and of course potions, but it was no use. The bastard is flying around with Quen most of the time. If I manage to hit him a few times, he will Aard me away, then set me on fire, and for good measure throw bombs at me too -- all within literally seconds, with no chance to roll away due to still getting up from the ground. Even with Quen most of the time, with these quick-fire combos, he will gradually wither away my health, and it's just a matter of time before I die.
After dying a thousand deaths, I did the only sensible thing and turned down to Easy, to get past this immeasurably stupid cheatathon. It's hands down the worst boss fight I've encountered in any game -- and that is definitely not a good thing.