I have not fought Jotun the hard way in a long time but there is an easy way and a hard. Unless rules have changed through time here it use to be that when you went to fight him it was easier to do the two sides of the dungeon first before going up the center to Jotun. I have heard that you eliminate some of the opposition in the Jotun fight if you do the sides first. The rules I read long ago went like this, When you enter the dungeon those with you had to be with you all through the dungeon, NO new players could enter once the dungeon was started. It made the efforts to remove some of the opposition at the Jotun fight null and void if anyone new entered the dungeon Later in play. If you left and used the commons button you could port back in. I was killed numerous times in a single fight with Jotun and went for Mana and new cards. If you go directly up the center be prepared to have a very hard fight on your hands and those with you. If things have changed on this dungeon and you know of changes please let players know because it use to be the worst quest in the game. I finally got where I refused to go in the place, as my life could not keep everyone alive and was being killed himself. Very tough place in its time ago, now? I dont know.........
The rules are basically how you stated, but there are a few other things I learned by farming those dungeons.
1. You don't need the same group on all three dungeons. As long as the people with you defeated the other two dungeons before entering the Hall of Valor, Jotun will be alone.
2. Someone who has not defeated the brothers in their separate dungeons can port to you in the Hall of Valor, but ONLY after you pass the door that is up and to the right of the entrance. If you cross that door when someone who did not defeat the brothers is in the dungeon, you will get the challenge from one of the brothers. If you get no challenge, then jotun will be alone.
3. Once past that door, you can not go back through it when there is someone in dungeon who has not defeated the brothers. If you do, then jotun's brothers will make an appearance.