Risk of Rain 2’s final boss is an entertaining fight, with some interesting mechanics. The first thing you will need to do is actually find the boss, which is done by coming across the Primordial Teleporter. This will whisk you off to the moon, for a jaunt towards a towering arena. When you get there, it will be time to face Mithrix, the final boss.
This guide is obviously a spoiler for the final fight, and we would recommend you experience blind at least once, as it is a lot of fun.
The first stage of the fight is quite simple. Matrix will charge at you, swinging his mighty hammer straight for your head. The best thing to do here is to distract him or keep as much distance as you can. Engineer turrets will tie him up for a while, and other characters can just kite him, as he will stop in place when he swings his hammer, giving your room to move. Just chip away at his health bar until he retreats to the center of the arena again.
The second stage will see lots of lesser enemies spawn in, including some flying ones. They can, if they combine their fire, do a lot of damage. Take them out one at a time, focusing on the ground targets first, as the aerial enemies tend to keep out of range and are less of an immediate threat.
The third stage sees Mithrix return, but this time he will be causing massive explosions of energy to erupt from the ground while he tries to hammer you into a pancake. Dodge these at all costs, as they do an extraordinary amount of damage and can easily kill you. When this round is complete, it is time for the most fun part of the fight.
Mirthrix will return to the center of the arena and draw all your items out of you. He now has all your power, and you are just a starting Survivor again. You want to instantly get within range and start pelting him with attacks, everything you can muster. Each time you chip away some health, you will get one of your items back, and you want to strip Mithrix of as many items as possible before he comes for you again. Eventually, the tide will turn in your favor as you will have more items than he does, and after that, it should be a simple process to finish him off.
Once the fight is done, you will need to escape from the moon by making your way back to the start of the level and reaching your ship before the moon can explode.