I had a feeling this will happen!
Demon Wall, while intimidating in its appearance, is not too aggressive in terms of damage. Demon Wall has a standard physical attack that is rarely used and only hits for approximately 100-200 points of damage. The real threat in this encounter is that Demon Wall constantly moves forward, eventually crushing the party.
The encounter with Demon Wall is timed due to his moving towards the party until they are crushed. A wise strategy would to be bombard Demon Wall with spells such as Blizzara from Rydia and using the Band, "Go Calcabrina, Go!" with Luca, Calca and Brina. Blizzara should do a consistent 500 point of damage while "Go Calcabrina, Go!" should be worth approximately 1000 points. If Rydia knows Bio she can cast that instead for even more damage, and no wait time on casting. If the player has any, it is also a good idea to use a Spider's Silk at the start of the battle to cast Slow on Demon Wall.
demon is "an evil spirit or devil, especially one thought to possess a person or act as a tormentor in hell."A