FL - 1 Tower Up Strat Guide.
Looks like a good guide and fairly comprehensive on towers and strats for each.
And this was interesting...
Leaving one Tower is the easiest of the hard modes. Frost or Storms is usually the one left up, as they are the easiest ones to avoid. No adjustments in the strategy are necessary, just make sure you move constantly and away from any beams.
Leaving two Towers isn't much harder than leaving one. Again, Frost and Storms is a common combination. If your normal strategy is not working well, you might want to leave one Siege Engine without a passenger, and put that player in a Chopper in the beginning of the boss fight. After you launch all your Demolisher passengers, the one in the Chopper will hop in a Demolisher, restocking it with pyrite for the entire fight. The Demolisher will keep shooting Pyrite for the entire fight, making the Leviathan drop very quickly. After an Overload phase, the Demolisher will shoot this passenger on top of the boss, and a new one will enter, doing the same thing - bringing Pyrite from the sky and refilling the Demolisher.
Those two are the ones you can cheat a little. You might want to shoot only 1-2 Demolisher passengers, giving yourself the opportunity to keep several 10-stacked Pyrite DoTs. This will involve a lot of pyrite consumption, but the boss will drop down very quickly if your Demolishers know what they are doing.