I have heard that jotun loses his brother's weaker if you fight through the side doors first, however do I need to fight through the side doors and then his door all in one session or can i do it over 3 different sessions?
Sadly you have to do it all in one sitting and, depending on if you solo it or not, it can take around an hour or two. Honestly I recommend just teaming up at the main door and hoping three other players join (that's what I did for two of my guys). May take a bit for people to team up but it goes by way faster in the long run.