Easier, less expensive way to kill him:
I feel like my method's more fun, but his takes less materials and less setup.
What you'll need:
- 3 Dark Swords (more if you're Wendy or Wes, bring six to be safe)
- 4-5 Logsuits
- Cane
- Healing items
- 1-3 Ice staffs (optional)
- Some kiting skill
Your inventory should look something like this.
Setting up an arena:
Dig out an 8x8 area with Klaus's sack in the middle and place down cobblestones, you can use the speed boost to dodge Klaus's lunge attack and the Deer's ice magic.
First Phase:
His first phase pattern's simple: swipe, pause, swipe, pause, deer magic (alternating between fire and ice each time). You can get two hits in between swipes. The deer won't attack you at all as long as you don't get too close to them.
When he uses the ice deer you can just run around with your cane and you'll either only get affected for a second or you'll dodge it completely, after you've escaped the ice you can get a few free hits on Klaus.
When he uses the fire deer you can freeze it with an ice staff and avoid the lava attack entirely, you won't be able to get any free hits on Klaus this way but it's a lot safer than letting him use the attack.
Sometimes he'll spawn a few Krampus, you can just kite them the same way you do normal Krampus, be careful not to let Klaus hit you while you're focused on them.
Second Phase:
Whenever he falls to the ground you should heal up if you've taken any damage or just wait for him to get up if you haven't.
His pattern's different now: Lunge, Lunge, Magic. Don't try to attack him while he's lunging, just run. Don't freeze the fire deer anymore since that'll just make the fight longer. And whatever you do, DON'T TRY TO TANK HIM, he does a TON of damage now.
After he casts a spell and you avoid it you can run over and get two hits on him, four if he does the roaring animation, but as soon as he's done you need to run so his lunge can't hit you.
After a bit of that he'll be down for real, the deer will break their chains, and you can claim the loot from the sack.