The thing is, this spell can effectively destroy many weapons and armor, and it can also ruin a lot of magic items (at least until the item is remade/repaired). it is almost like being able to throw a rust monster at anybody within close range. This is only a fourth level spell. I would place it as definitely better than rusting grasp (the closest similarly functioning spell of that level I could think of, even if it is a Druid spell), as it is ranged and outright eliminates armor, and, depending on a DM's ruling, might even end up turning into a much nastier tanglefoot bag when the metal becomes solid again. I do agree that it has incredible combat applications, but it seems a bit powerful, even as a 4th level spell. The only balancing factor I can think of is casting time, which is oddly absent from the text. And besides, even if we removed the combat applications with a long casting time, this spell has a buttload of usefulness built into it. That is why I think that it should be at least a full round casting time, if not longer.
