Miragul is a cure fight plain and simple. Chances are at level 80 you will need atleast 2 healers per group or a bad ass inquisitor with their mythical in one group.
Miragul has 3 very powerful AOE's that must be jousted by most everyone in your raid save for your MT's. Try to get all your resists up as high as you can. This will be a ranged fight for the most part.
Now, you will have gotten 2 items from the two trash mobs outside Miragul's room before the first trash named. These items banish Miragul's Guardian Dogs. One spawns at 65% and the other will spawn at 35%. Once one spawns you will need to use to the item on the three-headed dog. You only have 5-10 seconds to get the item on the dog before you will miss your opportunity. If you dont get it on him then it will become and immune and invincible. You also can not use the item right away either..it will take about 3 seconds for it to take effect. Just keep spamming it away. Do not bother putting a tank on the dog as you can't damage or taunt them.
Miragul also has two adds that come with the encounter at the start and then every 45-60 seconds. These adds have a massive stun. You will need a AOE tank to group these adds up quickly. The stun can be interrupted so scouts and enchanters better be interrupting and stifling these adds ASAP.
Miragul also has TWO curses to watch out for. One is a curse that if you use an ability will stun your entire raid for 5 seconds. Do NOT cure this unless you are a healer. Just leave it sit on everyone else except your tanks of course. Healers CAN cure themselves of this curse with out stunning anyone.
The second curse is a Charm.
Setup proper cure curse rotations and make sure you are communicating well incase a healer needs to cure themselves of the "don't cast curse". The detrimental icon for the bad curse looks like "the head of jesus"...seriously.
The fight is very linear and very scripted. You can time EVERYTHING fairly well.
Stay ranged as much as possible and if you can grab the adds and burn them down as fast as possible you will do very well.