It's one of those iffy calls. . .I'd go with Tim's 1 above.
It's easy to avoid, I'd even consider it "casual" non activity movement. (Wall is not thick enough to constitute 10% move for any but the slowest entity). . .I would force the combatant to select which side of the wall they choose to place themselves on with their casual step off the hot spot.
OTOH if you're chained to the wall and I choose to fire wall you while my henchmen fetch the BBQ sauce, you'd get cooked. . . .Then again, if I ordered my henchman to use the BBQ forks on you, I'd call that one of those "Medium moving maneuver to kill helpless target" results.
I have no problem with utility spells being almost unstoppably fatal when used in a really smart way under the right conditions.
Another example of that kind of U effect:
5 orcs chase melvin the magician up the narrow stairs of the tower to the roof, getting there ahead of the orcs, melvin finds the top of the tower a wrecked ruin of rubble. . .200# Melvin uses multiple Teleport spells to shift large 100-200# chunks of rubble so they fall down the stairs.
Now, in a narrow stairwell, the orcs are pulp unless there's some sort of freakish luck. . .no place to run, no place to hide. . .perhaps a roll to see if a rubble chunk jams in the stairwell or bounds overhead by freak chance.
I'd let a 101 ST fighter toss those same rocks down the stairs, so I have no problem with a mage doing it with magic.