It won't just be non-permanent, it won't even be on the majority of enemies you fight. Almost nothing is actually going to take the full 40 seconds that Melt Armor lasts to defeat, except EBs, AVs, and maybe particularly sturdy bosses, even without Melt Armor. You'll drop the mobs and move on to the next group which Melt Armor won't be up for, and it probably won't be up for the group after that, either.