At the bottom of this video will be a strategy video for the hard mode.
Ok for this boss its pretty fun, siege engine's are the first infront(not including choppers)
roles: Choppers will be gathering pyrorite and placing them near demolishers.
Siege Engines will be destroying towers and tanking adds such as collosal's and spider tanks etc.
Demolishers will be shooting stuff and killing stuff(also destroying towers)
The gunner of the Siege Engines will be taking out pyrorite balloons and copters
Once uve taken out all of the towers you will be going to the gate.
At the gate you will have adds, pull them away from the gate so you dont also pull the boss.
finish them off then back back and repair without pulling the boss, once all are repaired go ahead and pull FL(Flame Leviathan). You will then randomly be targeted by FL and that person will have to run away from him due to him hitting very hard and impossible to sit and tank it out.
you wont be healed in this encounter so its very important to not be hit by FL.
Flame vents are a channelled spell/attack and will need to be stopped by siege engines.
other then this you should be attacking him with everything you've got (whos not being chased ofcourse)
Do NOT shoot anyone on FL im not sure but this may start the hard mode but there isnt any need to shoot anyone on FL anyway.
pretty much run away if your being targeted and attack if your not. (choppers will still get pyrorite for the demolishers in this fight since there should be pyrorite shot down around leviathan)
here is the link for the strategy video on Flame Leviathan Hardmode: https://www.youtube.com/watch?v=JvraP9VCkyk&feature=player_embedded
Sorry about the spelling, tho it should be readable.