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.
This battle will be difficult, as the wall has 99,999 HP (in all other versions, it has 28,000 HP). Demon Wall will not attack normally as he did in previous versions. Giving Rosa the Dualcastaugment before the battle makes it slightly easier. Rydia should always use her best summons, and Kain, Cecil, and Edge always attack on their turns. When it is Rosa's turn, she should Dualcast Slow on the Demon Wall and Haste on herself, then cast Haste on everyone else. Afterward, Rosa should cast Berserk on Cecil, Kain and Edge. Rydia should keep summoning.
Killing this boss rewards an abnormally high amount of EXP points, even for a late game boss, which can help prepare for the end game.