Remember this is only done for 10 man. Not sure if it will work for 25 man.
To start , equip your gear with most item level. If you got Deadly Legs it's better to equip these than equiping Heroes Legs and so on.
Also you need to have atleast ONE caster to get this to work.
First talk to the guy in the middle of the entrance so you start everything. Then take two demolishers, one for each one of you.
Start clearing trash, destroy all towers etc. This may take awhile since you only are two peoples and two demolishers.
When you are at the boss kill the mobs infront of the gate and wipe when Flame Leviathan coming out, lol around with him a little if you want to.
Now when you're in again standing infront of flame leviathan. Take two demolishers, one for each one again. Drive so you're standing in the left back corner.
Shoot down some pyronite to the left side of you (this is the way we will drive when we kiting). You should shoot down atleast 3 pyronite barrels. And like a few yards from each one. One beside you in the start is also really really good.
Now it's time to play, you need to be fast after you shooted down the pyronite or it will despawn. Both of you in the demolishers, start fire of all your pyronite with pyronite barrels (or what the name of the ability is, anyways button 2).
Now when you're empty of pyronite in both of your demolishers flame leviathan is really close to you. Now you need to be quickly, the CASTER will need to jump out from his demolisher and jump in to the others demolisher, if it's two casters you choose who's jumping over to the other one.
Now when you're both in ONE demolisher the passenger need to pick up the pyronite on the floor quickly so you can get the speed boost, now you're out of pyronite again and need to pick up the other 2 pyronite a little way of the first one. You should now have 50 pyronite and are now ready to shoot away your passenger.
Passenger needs to load himself and the driver shoots him up to flame leviathan to destroy the turrents. This needs to be done quickly. If you don't got enough dps to get it down quickly enough try use some CD's, trinkets, flasks, pots etc.
Hopefully you succeded to destroy the turrents and now the CASTER(passenger) will be fired up in the air. Take of the parachute the fastes you can. The driver will now (when flame leviathan is out of date) spam pyronite barrels on him. When you're done and hes still out of date, use button 1 to do some extra dps. When the driver did this proccess the caster should have jumped in to the demolisher again and shooted down atleast 3 pyronites in the air infront of you, DO NEVER SHOOT DOWN PYRONITE IN BACK OF YOU, you can't drive towards flame leviathan to get some pyronite.. that's = doom.
Pick up the pyronite quick, make a speed boost, pick up the 2 other pyronites and repeat the things you did in the beggining.
Here is some videos that may help you, or to make things clear
From the drivers vision:
From the passengers vision:
That's all. Good luck and have fun. Hopefully you will get him down after a few tries.