On that link you can click on highlighted items for more information (like "prequests).
And I found it easier to do Nidavellir in three parts: Helgrind Warren, Winterdeep Warren, then the Hall of Valor. That way, I eliminated two of his brothers in the first two areas. So there was only Jotun and one of his helpers left. That made it much easier.
This link gives details on the three areas, and it has a good map for Winterdeep Warren that will help you avoid extra battles in that area. Don't forget to pick up the "book" in there too. Before you go "up the ramp" at the end to get Grettir, turn left and the book is in that area.
I believe Joturn is in Hall of Valor in Nidavellir. He is towards the back of the dungeon along with two other bosses and one assistant. It is possible to avoid the other mobs, but I suggest you bring help in order to complete the quest.
If you fight the other two dungeons in Nidavellir first and kill each of the brothers in those, you will fight Jotun without having both his brothers in the fight. It's much easier to do it that way. As for getting to him, no you don't have to fight anyone first. You can walk around them all if you're careful.