I beat the demon wall at lvl 17, although it took me a few tries.
My basic strategy was this:
-Make sure battle speed is HIGH - the wall won't come any faster, so you'll get more turns in.
-Equip Fran with Bow and Fire arrows, Basch with whatever sword (I had the blood sword at this point)
-Enter the dungeon and cast berserk and both of them.
-Walk down and begin the battle with the demon wall.
-Cast oil on it (If he has reflect, reset and try again)
-Watch as Fran and Basch obliterate it (have party leader be the 3rd person - I think the wall always x-zones the leader) Fran was doing 800+ damage each blow.
-If you're getting close and it still has a ways to go, Basch and Fran should be snapping out of their berserks.
-Use their quickenings to finish it off.

In my opinion, Berserk is the most useful spell at this point - they just sit there and attack anyway, so why not give them Haste and a Strength buff?