Link: http://www.wowwiki.com/Flame_Leviathan

Video: http://www.youtube.com/watch?v=hvPJCU83qXQ

Strategy
Normal Mode

After the last row of Storm Beacons there is a row of normal pillars. Beyond this is a roughly square room clear of all obstacles which contains a group of mobs guarding a gate. After this group is killed, the gate opens and Flame Leviathan enters. It is a good idea to kill the mobs from range, so that player vehicles are in a safe distance from Flame Leviathan when it enters.

While mobile, the boss always targets one Siege Engine or Demolisher and tries to ram it (which causes very high damage if successful). The targeted player must kite the boss, preferably using one of the speed boos abilities. The normal kite path is a wide circle along the outer edges of the area. Every 30 seconds, the boss switches to a different target, it also switches when its current target is destroyed.

Roles by vehicle type:

* Salvaged Chopper:

The main role of choppers is to drop Tar in front of the boss as much as possible. Dps boss with Sonic Horn only when Tar is on cooldown.

* Salvaged Demolisher:

Demolishers are ranged damage dealers. For normal mode, keep hurling Boulder at boss is good enough for the driver. Hurling Pyrite Barrel can do more damage but is not necessary. Since hurling Pyrite Barrel consumes pyrite, it is advisable for less experience group not to use Hurling Pyrite Barrel and save the pyrite for Increased Speed. The passenger can damage boss with the Mortar.
Demolishers should stay at range from the boss all the time, they are rather fragile and move slowly.
Demolisher gunners can simply shoot at the boss using the Mortar. When the Demolisher is chased and Flame Leviathan gets too close, the passenger should use Speed Burst.

* Salvaged Siege Engine:

Siege engines are melee damage dealers and interrupters. Stay near the boss and dps with Ram and Fire Cannon. The driver must take care to save some steam pressure to interrupt boss' Flame Vents with Electroshock.
When the boss is about to switch target (some addons such as Deadly Boss Mods give a 5 seconds warning), siege engines should start moving away from the boss. If a siege engine is targeted by the boss, it should start running away from the boss immediately. The driver can use Steam Rush repeatedly. When the boss gets too close, the passenger should use Shield Generator. Keep kiting until the boss change target.

Flame Leviathan is a very easy fight in normal mode. It is doable without the need to stun the boss (which will be explained in the next section). There is also no role / class requirement of the players and the gear requirement of the players is not strict. Therefore, it is possible to pug it.
Hard Mode

With towers up, the encounter gets increasingly difficult. To beat it, two additional elements compared to normal mode need to be considered: Shutdown and Pyrite. The fight can be simplified by just using Pyrite, but (depending on gear) it may be neccessary to use Shutdown, and then a rather complex choreography needs to take place.
Pyrite

Demolisher drivers can shoot pyrite. This causes a stacking DoT on Flame Leviathan, which at 10 stacks deals about 100-150k damage per tick. In hard mode, all other sources of damage can be ignored, as long as the Demolishers keep the pyrite stacks rolling. To make this possible, the Demolisher gunners must supply their drivers with pyrite. They must shoot it from the sky, and collect it with Grab Crate. The drivers must move so that they are close to pyrite on the ground.
Shutdown

Demolisher gunners can load themselves into the gun. The driver can then launch them so that they land on top of Flame Leviathan. There they can destroy turrets (2 on 10 player, 4 on 25). After all turrets are destroyed, the players are thrown on the ground and Flame Leviathan goes into an 8 second shutdown, during which it deals no damage, uses no abilities, takes 50% more damage and resets the Gather Speed buff stack.

The thrown players should be ranged damage dealers and (in 25 player) include one healing capable class (like an elemental shaman). Players do take some damage while on top of Flame Leviathan, it's not a lot, but still enough to kill a player now and then. On top of Flame Leviathan players cannot move, so only ranged can damage all turrets.

Shutdown is a highly desirable state, but shooting a Demolisher gunner reduces the pyrite supply for that Demolisher. A Demolisher can store enough pyrite for 10 shots. If the driver is conservative and shoots only once every 9 seconds just so the debuff stays up, the stock lasts a minute and more, which is in theory ample time for the gunner to destroy the turrets and resume collecting pyrite. In practise, drivers like to have a twitchy trigger finger, and collecting pyrite requires manoeuvering, so it may in effect take longer and the pyrite stack goes down.
Double-man Demolisher Guns

Another idea is to assign two gunners to some demolishers. Prior to pulling the boss, one player can preload in the demolisher gun, and a second player can man the gun. In that state, the preloaded player can be launched, while the demolisher retains a gunner to keep up the pyrite supply. After the first shutdown is over, the second player can load himself, and he can be launched as soon as shutdown is over, even before the first player is brought back to the demolisher.

In 10 player, one chopper is not used, and that player is used as second gunner in one demolisher.

In 25 player, various combinations are possible. If two damage dealers are enough to bring all turrets down fast enough, two choppers can be abandoned and two demolishers get a second gunner. If three damage dealers are needed, one chopper and one Siege Engine are left empty.