The trash before Flame Leviathan
Once you've got people assigned to vehicles, it's time to start your convoy. You'll roll briefly down a small ramp, and instantly start seeing dozens of mobs coming toward you.
Scattered around this battlefield, however, are many small towers. You've probably already heard that you activate Flame Leviathan's Hard Mode by leaving four towers standing.
The obvious, gray pillars in the middle of the field are NOT those towers. These towers simply must be brought down in order to stop the infinite waves of adds that keep coming. Each little tower is called a Storm Beacon, and each Storm Beacon is responsible for summoning the Iron Dwarf army you're fighting.
This part of the encounter always feels like controlled chaos, simply because of the vast numbers of mobs coming at you all at once. However, now's not the time to hesitate – all your gunners and drivers should immediately open fire on all the adds.
Move to each pillar in the middle and destroy it. On the outside perimeter of this battle field, however, is where the four towers live. Unless you're going to go for Flame Leviathan on Hard Mode, you'll need to circle each area and bring down the big four towers as well as the Storm Beacons.
There's two important tips for working your way through this trash. First, be sure to stay together. This lets you be certain all of the attacking adds are getting as much damage from the Area Effect powers as possible, and minimizes the amount of damage each of your vehicles will take.
Second, have your choppers repeatedly and quickly circle your convoy, laying down heavy barriers of oil slicks. Then immediately catch those slicks on fire. By now, most raiders are pretty well trained to "not stand in fire." Don't worry – the mobs have never had that training, and this fire won't hurt you.
Like I said, this part is going to feel chaotic. It's very difficult to see around a half-dozen of these vehicles moving in a tightly clumped pack. You're going to see an immense number of explosions, fires, and characters all around you. You might want to consider turning your graphics down a bit if you have too much trouble, but most groups don't really have difficulty with this part.
As you progress, you'll have two mobs in particular coming after you. You'll want to keep an eye out for these. The first is the Ulduar Colossus, who has about 300,000 hit points. He does a shockwave-like attack which Siege Engines can interrupt with their own Electroshock ability. The second is giant spider-mecha-gnome, which isn't quite as dangerous. Still, he's that big for a reason, and you'll want to focus your fire on him.
There are also helicopters that will come along and barrage you with lasers. You'll need to use your ranged attacks on these guys, as it's very difficult to try and ram them. These helicopters will keep spawning until you're in Flame Leviathan's final courtyard.
This trash hallway can take a little while to get through. If you find yourselves fighting for 20 minutes before you even get to the first boss, try not to panic. You're not necessarily "doing it wrong," but are just likely taking your time. Just be sure to destroy each Storm Beacon as soon as you can – the faster you get each beacon down, the faster you can get to the boss.
The other calming tip about this fight is that it'll sometimes feel like you're taking huge amounts of damage. If you have any kind of scrolling combat text showing your incoming damage, you'll see numbers zipping by incredibly quickly. Take it easy, and don't panic. Remember that your vehicle has easily twenty to thirty times your own hit points. These numbers are scary to you, but not to the tank you're driving.
A last note is that you'll find four glowing blue platforms along the side of the battlefield where you're fighting trash. These are the repair pads. If you run your vehicle over top of the pad, it'll heal you to full. Pretty handy, right? It does have a cooldown, technically, but I've never had a problem with it.