Pretty simple really. It's incredibly annoying that these two abilities don't share the same range, Molten Weapons has a much greater effective range while Obsidian Shard is stuck somewhere between melee range and Molten Weapons (which I think is where it should be. Bring Obsidian Shield and its morphs up to Molten Weapons' range, that way when I cast one I know I'm in range of my allies to cast the other. Never mind duration, Obsidian Shield is usually gone before the duration maxes out anyway, but the range does need to be fixed.