It is best to face Jotun without his two brothers at his side. In order to do this you must battle each brother separately Lokar (myth) can be found in Helgrind Warren and Gretlier (Life) is in Winterdeep Warren. If you defeat them first you only have Jotun and his minions to defeat in the Hall of Valor.
Tips and suggestions
Unlike other bosses, if you kill the minion first then are defeated, if you rush back to try again the minion will regenerate. You'll have to defeat both in one visit. The way most thought of, defeating Jotun first, does work but you either need to use a lot of Myth spells, or use traps and blades. It is possible to defeat them all at the same time by using spells like Fire Dragon, Frost Giant, Earthquake, Humomgofrog, and Meteor Strike. This has been done by strong teams without dying. A trick on getting rid of the Spirit Armor is to use Pierce. Jotun drops many good items, but the dungeon is to long and Jotun is to hard to defeat for farming. Be prepared with plenty of health and/or lots of life regenerating spells. Jotun has a high Power Pip percentage and will hammer away at you with Kraken and Storm Shark quickly & frequently.