Hello,
I've got strange problem from the very beginning of the first fight with letho my hp is going down wihout any reason even if letho is not going to hit me once, after 30 sec I'm dead- it doesn t matter if i used any potions or not. What should I do ?

did he chuck a fire spell or bomb at you? you'll catch fire and burn and drop health. But you shouldn't be dropping health simply from being in the fight..

Finish it faster. AArd him in the face, triple strong attack then aard again, repeat.