Hi, the fights are not that hard after figuring out an attack order that works for you. the most important thing is to notice when the monster is dealing it's big damage, which is every 2 or every 3 attacks, and to use your defender before it to block. other than that you need to have rations or else you wont be able to block.
the boss of chapter3 can kill you instantly only if you dont have a way to reduce it's rage, with seidr build i think its the asbru3, using the asbru invocation in the fight and enthralling the pup. using the potent huldra potion also helps with making hard fights easier.
if you encounter something that you find too hard in the game, join the discord server and ask there, you will most likely find out that you made some kind of mistake, such as forgetting to have a resource or doing something beforehand.