Yup. I loved the story and the cinematic feel of TW2, but I really disliked the combat. It's hard to be "under-leveled" for the main quest missions in any of The Witcher games. For TW2, it's ensuring you have gear appropriate for that stage of the game, then, as Riven says above, dive-roll, dive-roll, dive-roll... Blocking is pretty much ineffectual, unless you're going for a parry + riposte. (Dive-rolls.) Aside from that it saps your stamina way too much to be useful. (Dive-rolls into a dive-roll.)
For Letho in particular, I remember being really jaded by how grindy that fight was. I tried all sorts of stuff, and then basically settled on a pattern of using Aard followed by 2-3 heavy swings before dive-rolling away. Bubble, rinse, repeat for far, far too long. But it worked. (Also, don't attack him if he puts his Quen up. Use knives or bombs on it. Personally, I just activated Quen myself and waited for his to wear off. He's completely invincible while it's up.) You can riposte most of his attacks, and it causes a chunk of damage, but the timing required is crazy. Like Dark-Souls-1-fighting-Gwyn-with-a-regular-shield levels of crazy.