How do I defeat Mithrix?

Unfortunately the final phase of Mithrix is intended to be crazy difficult. There are some things to note though -

• His movement is slowed to a crawl in this phase so you shouldn't have trouble getting consistent hits
• As you deal damage, you steal your items back from him which can shift the power balance if you steal back some key items
• As long as you stay clear of him, his attacks in this phase are relatively slow so they shouldn't be too difficult to see coming and avoid

Obviously these points are very broad and swing very heavily depending on the character you're playing. A character with high base power like REX is going to have an easier time than a more difficult character like Mercenary.

The key things I've found to deal with the last phase are to stay clear, keep jumping and keep strafing. Eventually you'll get enough items back to take him down.

If you're really worried about the final phase and want to help secure the win, and are confident in your abilities before the fight, you could always take a few Shaped Glass so that when your items are stolen his health is drastically reduced... but with the nerf to one-shot protection in this version this is very risky unless you're extremely good at dodging every attack that comes at you.