I've only played through on one path on Normal so far, and I only beat Letho the first time by throwing bombs in his face until he went down. (Although that's one of those ones where when you "win" the game tells you that you really lost, IIRC)
One thing I found sad was that Roche's path is soooooo boring compared to... The one you took. >_>
I mean, I actually like him but his path was so much less interesting 'cause it was all humies.
Yeah, beating Letho on dark mode is the most annoying boss fight I've ever had. Wanna know why? Guess who can also use Quen? XD
It took me like 10 tries and I couldn't figure out why it was so impossible, then I found out that the mod I was using to make the game harder accidentally gave Letho super HP regen that made him impossible to kill with the gear I had. Once I disabled that it was about 5 more tries until I managed it. >_>
CEO and Better Combat for EE with Darkness I believe.
(It kinda does, but unless I'm remembering wrong you get him down to about 20% health and then the fight goes to Letho managing to hit Geralt with a good Aard. In that case it didn't feel disingenuous because you could understand Letho managing to do that, and you didn't actually beat him, just got him down pretty low.)