As if the attacking doors weren't enough...Rydia
This battle must be won quickly, as the Demon Wall will constantly close in on the party, and when it gets close enough, will start to use an Instant Death attack called Crush.
The player should immediately inflict Slow using either Spider's Silk or the Slow spell. Rosa should cast Berserk on Cecil, Kain, and possibly Edge if the player chooses not to use his Throw command. Rosa can then heal, or revive them with Raise if the Demon Wall uses its instant death attack.
Assuming the player has been to the Sylph Cave, they could equip Cecil or Kain with the Avenger sword. Not only does it put them in Auto-Berserk, saving Rosa a turn and increasing damage output, it also increases their Strength, Agility and Stamina by 10.
Killing this boss rewards an abnormally high amount of EXP points, which can help prepare for the end game.
demon is "an evil spirit or devil, especially one thought to possess a person or act as a tormentor in hell."A