Personally I found Isa no damage more annoying than challenging. Not to say it isn't challenging, it is.
There are many methods for ND Isa, down to no powers at all, (icebergs as projectiles). This is just my method, perhaps use it as a template for your own strategy or follow the video. Of the 6 bosses in Valhalla Mode, this isn't one you should worry too much about. Very telegraphed moves from Isa for the most part fortunately.
There are 13,822 views in 5 videos for Jotun. His channel published less than an hour of Jotun content, roughly 4.13% of the content that Dilated Pixels has uploaded to YouTube.