A master of illusion, Skitra delights in terrorizing his victims as he breaks their perception of reality. Those who fail to see though his deceptions become lost in Ny'alotha for eternity.
This fight consists of two phases, the boss phase and the illusion phase. Every 20% health the boss looses it will create 
Illusionary Projections of itself and each half of the raid (each tank will see different illusions) can see 4 of these illusions each. Two of these illusions will be on the same location so tank communication is required to uncover which is the real illusion.
Shadow Shock - This is the only ability the boss will cast on the tank, there is no melee swing. This applies a debuff on the tank which increases Shadow damage taken and deals damage to the tank + 2 random players. You should swap tanks when the debuff falls off (~4-5 stacks).
Shred Psyche - This is the main mechanic of the phase. The target of this debuff will have a 5 seconds to get into position, when this debuff expires an add will spawn and begin to cast 
Psychic Outburst which will deal damage based on your distance from it. After this the add will channel 
Psychic Reverberations which will deal Shadow damage every 1.5 seconds until defeated.
As shown on the map above you should place the add on the raid marker that is not on the boss, you should then move the boss on top of the add once Psychic Outburst has completed casting to avoid high damage on yourself and the Melee DPS.
[Heroic] Image of Absolution - A wall of Image of Absolution adds will spawn on one side of the room (as shown on the map above). If you stand on top of any of these images you will take high amounts of Shadow damage via 
Surging Images.
You are able to displace and root these images (same as Coven of Shivarra in Antorus).
After 30 seconds their 
Intangible Illusion buff will have expired, making them vulnerable to taking damage and can be removed from the encounter.
Your group will want to use their utility to keep the path that you are standing in clear of adds so you are able to negate the mechanic in it's entirety.
When the boss looses 20% health, it will transition into Phase 2.
Illusionary Projection is cast when you enter this phase. This will divide the raid in two with two separate debuffs, 
Clouded Mind and 
Twisted Mind.
Each half of the raid will see 4 illusions of the boss each. Luckily, the tanks are always given separate debuffs, so you can just communicate with each other whilst the raid waits patiently for the call on what to hit.
There are multiple strategies you can do for this phase, this example is what worked best for me in Mythic PTR testing. As shown on the map above, each tank will place 4 raid markers on the floor, where there are two raid markers this indicates that this Illusion is real. Likewise if Tank 1 can see a marker on an add that Tank 2 has placed they can call this out and place a marker down on it to help speed things along.
Each image will cast 
Illusionary Bolt which deals Shadow damage to all players every 2 seconds.
Every 5 seconds you spend in this Phase you will receive a stack of 
Dark Ritual, this debuff stacks and increases the Shadow damage you take by 2%. Your aim is to end this phase as soon as possible by identifying the correct Illusion as fast as possible.
Killing an incorrect Illusion will cast 
Mindquake which deals lethal damage to the whole raid.
The change for Mythic is that the raid will be given Clouded Mind or Twisted Mind at the start of every Phase 1.
Shred Psyche - The add that spawns from this can only be seen (and damaged) by players who match the person who received the debuff.
Image of Absolution - This will be assigned to one group of players and they will need to communicate where it is spawning from and how to move and that group of players needs to be ready to do Crowd Control for the raid.
Additional Note: For my Mythic test we stacked every Shred Psyche on top of the boss and nuked it down as we had a high amount of Mitigative cooldowns in our raid. Should you want to do the same the raid map of how this worked is linked here, and how our raid health looked from stacking the add is seen below.
Swap when your Shadow Shock debuff expires (~4-5 debuff stacks)
Be ready to mark adds in Phase 2 and communicate where necessary.
[Heroic] 
Ring of Peace is very useful to create a path for the raid on Image of Absolution
Fiery Brand will reduce the damage taken from Shadow Shock
[Heroic] 
Sigil of Chains is very useful to create a path for the raid on Image of Absolution
[Heroic - UNTESTED] 
Sigil of Misery can be used to stop the Image of Absolution from moving temporarily
Holy Shield is a must have on this encounter as there are no auto attacks; it's all magic damage
Blessing of Sacrifice can be used on your co-tank to reduce their damage taken
You are able to 
Spell Reflect to reduce damage taken from Shadow Shock
Rallying Cry is a powerful raid cooldown to use during Phase 2
Stampeding Roar can be useful to help your raid move towards the correct Illusionary Projection during Phase 2
[Heroic] 
Mass Entanglement is very useful to create a safe area for the raid on Image of Absolution
You are able to negate a stack of Shadow Shock with 
Anti-Magic Shell
[Heroic] 
Gorefiend's Grasp is very useful to create a path for the raid on Image of Absolution