The fight with Ivarr, despite his fame, will be much like any other boss fight. The major difference here is that Ivarr has the fastest reflexes you are likely to have seen so far. As a result, the skill Brush with Death will be extremely useful, allowing you to dodge and slow time enough to consider your next move. Most attempts to hit Ivarr with a basic attack will be easily dodged, so you’ll have to mix things up a bit.
Ivarr’s basic attack will involve a number of swift blows with his axes. He’ll sometimes follow this up with an unblockable / unparryable attack, marked by a red rune aura, so be sure to move out of the range of this with a dodge or roll. You’ll find Ivarr’s weak points on his right leg and his left arm. Be sure to hit these to make him vulnerable to a stun attack, which will take a large chunk of his health early on in the fight.
When Ivarr is standing still, he will brush off most arrows easily, but he will be vulnerable as he’s charging you. During a charge, he’ll move from left to right very swiftly, then continue head on. Don’t bother trying to shoot at him until he’s performed these movements, unless you get lucky with an arrow you’ll find it difficult to hit him. The best time to shoot is immediately after he’s moved left once and right once, but you’ll have only a brief moment to land a hit before he reaches you at this point.
If you create some distance between yourself and Ivarr, he will sometimes throw some axes at you. These are easily avoided with a simple dodge, but you should still be prepared for them whenever there’s a reasonable distance between you.
When Ivarr has around a quarter of his health left, he’ll sit down and ask to speak with Eivor. This is a trap, so be ready to block, parry, or dodge at any moment. The initial attack will be followed by a red rune aura attack, so you’ll need to dodge or roll to escape this. During this last quarter of Ivarr’s health, he will become much more aggressive and attack much more frequently. This is a good time to use your high damage abilities to offset this assault. Aside from the increased attack speed, there will be no new attacks during this phase.
When the fight ends, you’ll have to decide whether to send Ivarr to Valhalla or not. It doesn’t matter what you choose here, so just go with whatever feels right to you based on how your interpret Ivarr’s actions.
What to Tell Ubba
As this fight is part of King Killer, you can check that page to see what you should tell Ubba to maintain good relations with him. However, in essence, you should tell him that Ivarr died badly, which in the eyes of Eivor and Ubba is the truth.