fireball and lightningbolt are, as one would immediately expect, taken. However, if you must have an attack spell username, "coneofcold" is stll free; and, somewhat surprisingly to me, "magicmissile" is available.
flamestrike, which is what I was thinking of in the first place, is a deleted journal. polymorph is taken. Oddly, "geas" is not. Nor are "cloudkill" and "dancinglights", both of which seem entirely worthy of use.
I tried looking up "teleportwithouterror" (teleport, check) and "circleofprotection", just in case, but LJ tells me "malformed username." Perhaps there's a character limit the longer spell names exceed?