It's a defensive more than an offensive team.
Don't pay too much attention to the fire. Use the "push" effects above all. Only aim for direct damage when you have an opportunity, or when an enemy has only one life point (burning, they're dead-men-walking in the absence of regenerative "psions", as fire effect is the first event calculated). Aim for survival (of your mechs and of the buildings) more than for elimination. Avoid missions that ask you to destroy stuff of creatures.
Sometimes you'll clear the map, but often not, and it shouldn't be the goal. Set the land on fire (especially the emerging points), and just consider that time is on your side.
Put points in the length of your science swap ray, and exploit the fact that your science mech hovers above water, lava, and pits. Later, put points in your gunner mech, to add damage to it, but don't change your strategy too much.
Remember that your flamethrower only damages already burning targets. Remember that your gunner (before being upgraded) won't damage your units or buildings by firing on them (and pushing the surroundings away), so use this while you can. But remember that some of the things you're supposed to protect (such as rockets on the launch pad) are susceptible to fire.
Basically, it's a corrida. Only exception : the floating psion creatures, which should be actively destroyed as soon as possible, when possible. Because they tend to counter the fire effect (by healing the burns), or to provoke random detonations (with burning creatures dying on their own, and behaving like ticking bombs around the map). But apart from that, just let the environment become too hot for the creatures while you just use your tools to keep the mechs and buildings out of harm's way.
