What Elemental Mastery: Fire and Earth does is unlock some extra available Actions for each eligible Supreme. The wording of Collapse allows that it is a cheap way of Damaging buildings for Stoner Hawk (think Destroy in the Destroy or Defend Plot). Smoke allows Sgt. Bale to protect his Team-mates - especially those with low Agility - from long range attacks (more than 8 inches), since it moves with him; definitely worth considering if likely facing Gentleman with Sniper Rifle, Virgo, Vector or Xyllian, Master Sagittarius for example.
However it is Lava Flow where Elemental Mastery: Fire and Earth comes into its own. It can be expensive and is best used either early-game, or if your enemies are clustered up and fairly close by. Potentially very destructive to terrain, Citizens and sundry others (Destroy or Defend again, Invasion), it can also offer a creeping area denial. Do not consider it as a targeted attack (its movement is too random after all), but rather use it to imply a threat, which is where can wok as area denial (useful in Take and Hold) - especially against Water Supremes and Minions.