Shabu and I (Koro) are two 5-boxers. Basically our setup was to have 1 Siege, 1 Demo, and 1 Cycle per group. That way everyone is in a vehicle from each group.

We placed the siege engines in the southeast and southwest corner (right in the corner with no extra room). Shabu and I manned the driver seat of a demo so as to maximize dps. The cycles we kept on follow for the most part. The demos roamed the field freely.

We placed the siege engines in the corners so as to distract XT from the demos and keep them safe for most of the fight. We did not put any dps up onto the actual Flame Leviathan, so we got the Shutout Achievement. We also didn't shoot down pyrite. When FL chases a demo, we lay down tar and run away. This ensures that there is a constant stream of dps on FL at any time during the fight.

The best way to approach this fight is to start out with the two siege engines in position, and just wing it from there.

Tip: When he first selects a person to chase, switch to the motorcycle alts and lay down tar in front of him so that he doesn't actually hit the targetted person. The first targetted person (if they're a demo) should stay in the very back of the corner with the siege engines. If he switched to the siege engine already in the corner, it's no problem. They're basically there solely for distracting FL and soaking damage... It doesn't matter if they die.