Everyone knows how to do Flame Leviathan, I am not going to explain the basic strat, I am going to explain things that are VITAL to getting a two or three towers under control. Four towers is an entirely different matter, that will have it's own strat post.
There was a lot of confusion about "hard mode". When you start, you can initiate the event by talking to one of two people. If you talk to Brann, none of the orbital defense systems are activated. This is Normal Mode.
If you talk to Norgs (as I affectionately dubbed him) he sets off the towers. All of them. As you move through the gauntlet you DESTROY the ones you do not want to do, leaving up the ones you do.
The towers we are currently doing (in order of preference) include:
Tower of Storms (Thorim's Hammer) – deals Boosts Flame Leviathan's Physical Damage by 25%. Shortly after the fight starts, Flame Leviathan will activate Thorim's Hammer. Thorim's Hammer causes pillars of light to appear and shortly afterward do around 10% (150k or so) damage to any targets under the beam along with a 1% damage AOE to all vehicles. Total of about 20 Beams. Flame Leviathan will activate this only once. Just, dont stand in any beams and you will be ok.
Tower of Flames (Mimiron's Inferno) – Boosts Flame Leviathan's Fire Damage by 50%. Shortly after the fight starts, Flame Leviathan will activate Mimiron's Inferno. Mimiron's Inferno causes fiery orbs to fall from the sky in a slowly set path. Orbs start where Flame Leviathan entered the room and continues in a clockwise direction in a diamond formation. Orb strikes deals fire damage to all nearby enemies, leave behind a flaming blaze that deal damage over time when touched. Effect lasts for the entire fight but are pretty easy to avoid.
Tower of Frost (Hodir's Fury) – Hodir's Fury cause's several White-Teal beams to appear and follow players around. The beam itself does no damage but after it stops moving on top of a vehicle, it will call down a ball of frost that deals frost damage to all nearby enemies and entombing them in ice where they take an additional 1% damage per second. Ice is broken by a fire attack. Effect lasts for the entire fight. Simply move directly forward or backward when you see the beam"lock on" to you and you will avoid this.
The above three towers are pretty easy mechanically. One leaves a trail of fire you need to avoid (easy if you pay attention), one has big beams from the sky that chase you around and one puts out a singular random event of big blue AoE bolts from the sky. Save shields for this event. Apart from that, it is business as usual, except FL has more health, so you just need to be good at not taking damage and good at dealing it.
The key to this fight are the shutdowns.
Send 2-3 people up as soon as FL is engaged. They are marked, their vehicles are marked, and we have allocated a chopper to each player to pick them up and deliver them back to their vehicle after shut down. They then quickly grab some pyrite and are thrown up as soon as possible. Getting this cycle right, ensuring there is minimal down time and that the people do not die is the way to kill this boss.
The 3 people are 2 DPS and a DPS off healer (like an ele shammy, they do this the best). If you have no healer up there you will die. It's not a lot of damage but it is enough to kill most guilds in Naxx or early Ulduar gear. You NEED to have ranged go up as when you are on top of FL you cannot move. You are thrown to a position with a tower to kill. Kill your tower, then target another tower and kill it asap. Once all 4 towers are destroyed, FL shutsdown and the DPS are flung off the top of the tower.
Once Flame leviathan has finished repairing his towers (the reason for the shutdown in the first place) he comes out of shutdown and activates again. As soon as shutdown is over, you can send up your team again to repeat the process.
DPS during a shutdown
This is where about 80% of your damage will come from. FL takes extra damage while in shutdown - take advantage of that.
Demolishers must be fully stocked or close to it (it takes 2 pyrite to fill the tanks), and they need to launch all their pyrite at FL during this phase. Those pyrite launchers do stupid amounts of damage. It doesn't matter if you dont DPS any other time (apart from keeping tar alight) your main job is to make sure all that pyrite lands during shutdown. If you are a passenger who is not on the tower, keep stocked with pyrite, mash your rockets (2) and keep tar alight (1).
Siege engine passengers should be keeping the odd tar alight with (1) but mostly mashing (2) as that is a very powerful attack.
Choppers get that tar down, and keep laying it! If you get bored, grab some pyrite and shuffle it near the demolishers.
Doing all of the above should get you a really good dose of death on FL.
What to do when NOT in shutdown
DPS is not so important at this point, but not unimportant, if that makes sense.
Demolisher passengers (once back in the vehicle) grab pyrite asap. Those not getting flung should be shooting down pyrite for the flung ones to quickly retrieve. Once they are done, load yourselves up. Keep tar alight, and DPS when you have time. Your job is to stay as alive as possible. Stay at range.
Siege engines get in and interrupt the flame jets. Passengers keep mashing (2) as you are the main DPS dealers for this phase of the fight and only hit (1) when you see unlit tar. When the 5s warning comes in your fave boss mod, all siege engines get away from FL so that you have a good head start on kiting if you happen to be picked. If Thorim's hammer comes, make sure you have a shield ready (1 min CD). Passengers, if you can, make sure that you shoot down pyrite when possible as well!
Choppers will pick up their person, and return them to their vehicle as fast as they can, then back to dodging around and dropping tar. Again, try shuffling pyrite into the middle to help the demolishers.
What to do when kiting
Run away. Stay as far away as possible. Passenger should be liting tar as they go. Kite the FL through tar, I cannot stress how important that is, and make sure that tar is lit up. Avoid the orbital defense fires, try not to get trapped by them. Easy *cough* yes, easy!
That's it. Rinse repeat. Make sure that you take as little damage as possible, and you maximise DPS during shutdown. It should take 2-3 shutdowns to get the boss down depending on how well you kite him through tar at other times.
This applies to 10 man as well, just fling 1-2 people not 2-3!
Practise your shutdowns with two or three towers up ready for four towers. Two or three towers have been nerfed a fair bit recently, and they are a lot easier now than when this guide was first written (We found that 4 towers now is as hard as 2 towers was back in 3.1.0). Yes, you can almost certainly Zerg through 2 towers now with no shutdowns. Three you might be pushing your luck, and doing 4 you definately need to shutdown, so you may as well get some pratcise in. Getting the shutdowns means the fight goes a lot quicker as well leaving you more time for working toward other hardmodes and Yogg!