Looking at one of three attacks for a Fire/Earth

Currently moving along on my latest Dominator, into the mid-20s and regardless of anything else, I'm really loving the SMASH-y feel of Earth Assault. Based on hearsay, though, I've pretty much given up on trying to stack Mud Pots with Hot Feet, and that's left things open for me to include one more filler attack to use.

I'm trying to decide between Ring of Fire, Hurl Boulder, and Air Superiority. All of them are nice for keeping enemies on the ground, and I can see distinct advantages and disadvantages to each of them. RoF and Hurl Boulder will give me range, and be helpful with enemies trying to escape, but the former negates /Earth's knockdown while only doing its damage over time rather than up front, and the latter does knock BACK, unlike my other attacks, which is a bit of a pain, since I'm mostly melee focused. AS lacks these issues, but it's yet another melee attack, and it'd be nice to have something extra to use against enemies that try to run (Spears works fairly nicely, unless I'm fighting Longobow Eagles or Freak Juicers, since they prefer to FLY away).

I'm fairly torn about which to use. I don't really need a filler attack - perhaps that could be seen as Stone Spears offering a blessing in disguise with its general slowness - I'm looking for something I can use to finish of weak enemies and, again, keep fliers from causing me too much trouble. I'm curious what everyone else's experience would suggest.

Air sup fills nicely for the other melee attack that they forgot to put into earth assault.