EDIT: Learned a few things about the fight. Additions will be highlighted.
This is not meant to be a definitive guide to killing Tiamat, simply some basic guidelines to help people complete the encounter. I am sure many tricks and tips will be learned over time, but as of my 3rd attempt it is obvious that some information needs to be shared.
Tiamat is an instance containing 5 groups of players (25 people total). We are required to do several things to achieve victory:
1. Kill summoners
2. Kill Severin
3. Protect Clerics
4. Attack a head of Tiamat
5. Protect Clerics
6. Attack heads of Tiamat.
(Cleric and head phases repeat)
In order to do this correctly:
1. Form into 5 groups. Each group MUST BE FORMED ONCE YOU ENTER THE ANTECHAMBER. Currently pre-made groups have no guarantee of being in the same instance, so wait to form your groups until you have entered the antechamber. You have 4 minutes (maybe 5) once you enter until the fight begins. DO NOT WASTE THIS TIME. There are 5 colored altars in the antechamber. 5 people should go to each altar and form a group. The color of altar will determine which head you will focus on for the rest of the fight, and which summoner you kill and which gem you collect. THIS IS IMPORTANT. 5 people per head.
2. There are 5 Summoners, one for each color of Tiamat's Heads. Once killed each of them leaves behind a clickable icon (looks like a Wizard's Artifact orb) which will grant you a colored gem. A player my only have *1* gem at a time, each gem provides some protection to the breath weapon associated with it's color. Once you have a colored gem, YOU SHOULD REMAIN FOCUSED ON THAT COLOR HEAD FOR THE REMAINDER OF THE FIGHT. After your Summoner is dead, pick up the gem. THEN PUT THAT GEM IN YOUR POTION BELT FOR QUICK USE. The gem functions like a protection spell, lasting about 10 seconds with an approx 2 minute cool down. The dragon head breaths about one time every 2 minutes. Use the gem to protect yourself. Once all 5 summoners have been killed you should fall back to the center area to kill Severin. He is an easy kill, so you do not have to fall back if you have a slow mount or are just feeling lazy. Just as long as he dies quickly, we are all on the clock in here.
3. Each group (which you formed before the fight) should focus on a single color. The color you focus on will be the color of altar where you formed the group. That means that when the fight starts you kill your color of summoner, get your color of gem and then help kill Severin. Then you go back to a Cleric near the head you will be fighting.
Black is the farthest to the left. Stay near your cleric at all times. Green is next, Green players should split time betwen the Cleric near the black head and Linu near the middle. Red Players stay near Linu for the entire Cleric Phase. Blue players move from Linu and The 3rd Cleric. White Players stay near the 3rd Cleric for the entire fight. EDIT: Black group protect the cleric far to the left. White group gets cleric far to the right. BLUE RED AND GREEN FOCUS ON LINU. Only leave Linu if you think black or white is in trouble. LINU GETS 3x AS MANY ADDS, 3 GROUPS OF PLAYERS NEED TO DEFEND HER. If you see that Linu is well protected it is reasonable to break a couple of people off of her to assist either black or white, but only if Linu is OK.
4. Once the clerics have done their thing everyone MOVES TO THE HEAD THEY HAVE CHOSEN. You will have some protection from the breath weapon as long as you picked up the gem. That does not mean you are immune to it, but it will be survivable. Do as much damage to your head as possible. BUT DO NOT KILL IT, DO NOT KILL IT.
5. To finish the encounter all 5 heads need to be dead at the same time. EVERYONE NEEDS TO PAY ATTENTION TO THE HEALTH OF ALL THE OTHER HEADS. When all heads are close to death you then kill your head. Once your head is dead feel free to assist another group in killing theirs. Rinse and repeat.
Coordination is important in this fight, but it is not hard to do once everyone knows how it works.