This boss is divided into 4 phases. Mimiron will use a different vehicle unit in each phase for phases 1 to 3, and in the final phase he will use all 3 units together, which they all must be destroyed within seconds of each other or they will self repair.
In the first phase, Mimiron will use the Leviathan MKII, a siege tank like vehicle. In this phase there 2 main abilities to watch out for, Plasma Blast and Shock Blast. Plasma Blast is a channeled DoT that deals heavy damage every second for 6 seconds in addition to his melee attacks. This should be dealt with by using a cooldown such as Dispersion. Healing through the Plasma Blast without any cooldowns does work, but health must be relatively high beforehand. Mimiron also uses Shock blast, which is an ability that deals heavy damage to anyone within 15 yards, and this can be avoided simply by running out of range.
The second phase begins when Leviathan MKII is down. Mimiron will now pilot the VX-001, an anti-personnel assault cannon. This phase does not require a tank hence incoming damage is minimal. PW:S is all that's needed for this phase. However there are still a couple of abilities to watch out for. The first one is Rocket Strike, which deals lethal damage even at level 85. The ground which this will hit will glow beforehand and there is plenty of time to move out. The second ability is the Laser Barrage, VX-001 will take 3 seconds to charge up the laser, the fires barrages of laser for 5 seconds or so, dealing heavy damage to anyone standing in its frontal cone. Aside from these 2 abilities, phase 2 is relatively easy.
Mimiron will then pilot the Aerial Command Unit in phase 3. During this phase he does rather little damage but will summon Assault Bot and Junk Bot adds to attack. Unless your ilvl is through the roof, it is not recommended to ignore these bots. In addition, The Aerial Command Unit itself will drop bomb bots and these run towards the player and detonates. Again, it is recommended to kill these bomb bots ASAP. The last significant issue regarding this phase is the occasional dispellable debuff which has a root effect. While this debuff is active, all damage received will be increased by 30% so dispel this debuff ASAP.
Once the Aerial Command unit is down, phase 4 begins. Mimiron will then combine all 3 units together to form a super robot V-07-TR-0N. The abilities used includes most of the special abilities used in the previous phases. This phase is actually easier than it seems as using a special ability requires charge up time, and that is the opportunity to heal up or make distance. V-07-TR-0N will still melee attack when not using any special abilities so this phase is still healing intensive. Let dots do most of the work here. The 3 components does not all have to die exactly at the same time, but they should die within around 10 seconds of each other. Live through this phase to win.
PS. Yeah I got Laser Barraged to the face at the end... surprised I survived. But this video is actually my first time soloing the final phase. Most of my unsuccessful attempts was on phase 3.