First you will be taking out the 5 channelers around Mag. You can walk into the room without argo, just dont attack anything yet. There will be a tank, a healer, and a cube clicker assigned to each of the 5 cubes. Fight all of the channelers, one after another, until all of them are down. Then run to your cube clicking location.
You will want to stay somewhere near your cube at all times so you can quickly get back. This is a fight where addons like BigWigs and DeadlyBossMobs is VERY handy because you can see the cooldown of Blast Nova. Whenever you see his casting Blast Nova click on a cube and you will channel a beam to prevent him from casting. You will take damage while doing this so your assigned healer should be nearby. While all 5 of the cubes are clicked Mag will take 300% damage so if you are not a cube clicker lay on the DPS during this time but dont go over the MT threat.
Be careful about the Quakes because they will bounce you around for 7 seconds. You will want to head to your cube about 10 seconds before the Blast Nova cooldown is up so you can be sure you will get there in case a Quake happens.
Watch out for patches of fire on the floor. They do 6k fire damage over 8 seconds if you go through one.
Mag enrages after 22 minutes and will wipe the raid.