You will fight Mitra in the eighteenth and final level 'The Monolith of Inhumanity'.

The fight itself will pivot between fighting Mitra in the arena you fought Ahriman in and within Cybervoid. In the former you will have to survive by jumping over shockwaves, dashing away from projectiles, and finally parrying his attacks. Which, notably, is the only way to deal any amount of significant damage. If you're not great at parrying don't worry, as half of these attacks will be free parries; pausing the game right at the parry window to allow you to get a hit in.

After dealing enough damage to him, you will be taken to the Cybervoid. From here you will have to traverse over to him whilst dodging a host of projectiles. Once you do, you will be taken back to the arena and the cycle will repeat until he is dead.