This guide will assume you are familiar with the Heroic version of the fight and expand on how the fight changes in Mythic difficulty. If you are not familiar with this encounter, check out our heroic guide by clicking the button below.
Mythrax has one substantial change in each of his phases on mythic difficulty which is the addition of his little squid monster Xalzaix. In phase 1, Xalzaix joins the fight every minute. While above 50% health, it casts Void Echoes and Obliteration Blast in a cyclic pattern. Every successful cast of Void Echoes increases the damage of Void Echoes by 50% until Xalzaix is brought below 50% health which causes him to retreat back to Mythrax. In the intermission phases Mythrax retreats to his starting position rather than the center of the room. This is because Xalzaix flies to one of the other three corners. Mythrax then aims his Obliteration Beam at Xalzaix who redirects the beam to a random non-tank player. Bait this by stacking all your ranged players together and move between two points every time a beam is cast.
Due to the spread nature of the fight, Wellspring has diminished effectiveness, so High Tide is the go to talent. For tier 90 talents, your only options are Flash Flood and CBT. If you take CBT, time it so you have no chance of healing Mythrax if you were to get mind controlled by an Oblivion Sphere with the explosion of CBT. The last option for talent swaps from this setup is Echo over ES. ES is strong on this fight due to generally needing one healer to be with the second tank away from the group during intermissions. Shamans are great at this job due to having Wind Shear and ES helps your lack of single target healing on that tank.