I enjoyed this (and am sure some hate it) - because its nothing like WoW. I could take any computer game savvy person, and they could perform as well as a hardened WoW veteran on this fight.
I won't lie that a decent amount of this fight seems random-esque. Not coin-flip win-or-lose, but there are tons of factors that can make things rough one attempt, and easy the next. That said a group of people with a solid strat, and some dedicated time, can come out on top (and collect three conquest emblems, three pieces of loot, and a good chance at a runed orb / pattern).
How to do it
I'm going to assume you've done FL before, and have basic knowledge of vehicle abilities. FL himself will act the same, but with greatly increased health, increased damage with his normal attacks, reduced damage taken from your Pyrite, and (of course) the addition of four orbital defense systems.
Vehicle Setup (at start) :
Demolisher with Driver & Co-Pilot
Demolisher with Driver, Loaded Passenger (Non-Hunter Ranged DPS) & Co-Pilot (Non-Hunter Ranged DPS)
Siege Engine with Co-Pilot
Siege Engine with Co-Pilot
Hazards All Drivers Must Deal With
Thorim's Beams: These are the stationary white/blue beams. They'll appear randomly scattered around the map, twice during the fight. When they activate, all vehicles will take some damage, but vehicles close to them will take far more. Note that these two beam activations take place early on in the fight, so it will seem like you've taken a ton of "unavoidable" damage (around 40%) early on - but when the second set of beams is done, your incoming damage should drop off dramatically.
Mimiron's Fire: This is a orange beam that leaves a trail of fire. It travels in a diamond shape around the map, and can easily trap someone in a corner. Its easy to avoid damage from this beam in general - its true danger is in limiting your escape options from other hazards.
Hodir's Beams: These blue/white beams are "random", and highly annoying. Each beam will pick a vehicle and attempt to move on top of it - while these are moving, they pose no threat. When the beam stops, however, it starts to send down a ball of ice. If this lands on any vehicle, that person is frozen - taking a large amount of damage until they are broken out with a siege cannon blast, burning tar, boulder, or pyrite. The beams can change targets at any point.
These are most dangerous to Demolishers, that need to keep FL in their sights / stay by pyrite, and are therefore most hurt by needing to move erratically to avoid being frozen. Forward movement will always get you out from under them in time, but reversing can sometimes get you caught.
Freya Adds: These spawn in the corners periodically. Each wave has several small ones, and one large lasher. Left alone, these can take out vehicles rapidly. Drivers with adds on them can attempt to drive in/through lit tar, to damage them (though this isn't required).
Jobs by Vehicle / Position
The Demolisher Driver with just One Co-Pilot has a straightforward job: keep ten stacks of pyrite on the boss. Get ten stacks on him right off the start, then switch to weaving in boulders to conserve pyrite (since its the tick of the pyrite DoT that deals damage, not the actual landing of the pyrite on him, and the DoT lasts somewhere around 6-10 seconds). I alternated one boulder : one pyrite during times that I had to move lots or was far away, but occasional was able to do two boulders : one pyrite when I knew the last pyrite would hit in time to keep up the stack.
Keep in communication with your co-pilot, as to when the area around you is running out of pyrite. You'll want to have a full tank *before* you start moving, so you can keep applying it as you move.
The Demolisher Driver with a Loaded Passenger waits for FL to pursue his first target, -then- launches the first Non-Hunter Ranged DPS. Shoot earlier and the person will fly right past him. After they are on FL, act like the other Demolisher (above). When FL finishes overloading, your current co-pilot (the other non-hunter ranged DPS) will load himself into the catapult, and get fired as soon as FL starts pursing the other person. You'll be without a co-pilot for a short while, while the chopper driver drops off your previous passenger.
Demolisher Driver Co-Pilots primary task is keeping their demolisher stocked with pyrite. This is half actually targeting and reeling in barrels, and half communicating with your driver. Your driver needs to know when he can sit in one spot and blast away, and when the surrounding area is empty of pyrite - and they need to start navigating elsewhere. Giving speed boosts is important as well - you'll want them when FL chases you, or when you need to get to a fresh pyrite-containing corner - but your use of speed has to be carefully measured due to its cost. During periods that you aren't scouting for / picking up pyrite, shoot down more of it.
For the two People Being Launched - you'll want to let the first person being launched get in first, and then get into the catapult. When you get shot up, start going to town on the turrets. Call out when you estimate that overload is ten seconds away, and then five seconds away. After you get shot off, look for the chopper driver, and GET IN. Any of FL's abilities will kill you, including flame jets (if he was casting it when he went into overload).
The person remaining in the Demolisher should not load themselves until the overload is over - they should remain in position to collect and shoot down pyrite until ready to be shot. When they load themselves, they should call it out - this is the cue for the other person who's passenger in the chopper to jump out, and immediately into the demolisher. If they jump out before the other person is in the catapult arm, they will not be able to get in - and die.
Siege Drivers have an un-complicated job. Don't get melee'ed by FL, ram adds when possible, and interrupt flame jets. Melee'ing FL isn't worth the danger associated with it, and losing a Siege causes Demolishers to be pursued more (and do less damage) - so don't sweat it. When there is five/ten seconds left till target change, back away from FL. Consider that the target person can then steam rush away, and the one not targeted can rush towards him and int erupt (if needed).
Siege Passengers have an un-complicated job as well. First priority is Cannon Blasting Adds - using mini-missiles on them (or to ignite tar) while its on cooldown . When adds are dead (and they come in waves, so there should be down time) - shoot down pyrite. While FL is overcharged, if there are no adds - you can shoot at FL, but this should be the only time.
Chopper Driver has lots to do. Sonic horn grabs aggro on adds extremely well, and when combined with speed boosts, lets him kite adds around. Laying down tar snares adds, and burning tar damages them also. Adds left on a siege or demolisher will kill it in short order, so protecting them is key. As a bonus, you get to pick up the ejected person during overloads, and ferry them to the demolisher they were shot from. Make sure you raid mark the two people being launched so you can find them easily.
All Passengers: Pre-Shoot Pyrite Barrels from the sky before each pull.
Shoot down pyrite before pulling. Having tons to start out with saves you time during the actual fight. Make sure to get 7+ canisters landed in the two close corners (as you look at him), and lots in the back right. Back left corner just doesn't have much ever fly over it.
If things go well, you'll go through two to three overloads before he bites the dust. Each job is simple on paper, but performing them correctly while simultaneously avoiding hazards takes some practice. Make sure you set aside a large chunk of time for people to master their jobs, then keep plugging away. Sometimes FL and Hodir's Beams will target your demolishers constantly (bad), and sometimes they'll stick to siege engines (good). Things can go wrong at any point, and sub 10% wipes are very possible (and likely). You won't know the fight is really over until your vehicle falls apart underneath you.