Wall of ice is the best wall spell, hands down. It doesn't need specific supports like wall of stone, it is solid unlike wall of fire, and it can oriented in any direction unlike wall of force.
Monster Tactics: The osyluth. Wall of ice at will. Caster level 12th, so 12 10-ft. squares or a hemisphere with a maximum radius of 15'.
These tactics assume a cave or lair of some kind, not a wide-open flat plain.
Tactic 1: Cast WoI (plane) about 1" off the ground, creating a new floor for rogues to slip on and to force normal movement. Use at will fly to stay above it.
Tactic 1a: More generally, use the plane form to make new surfaces to fight on, or force people to fight at a disadvantage. Sloped floors, etc.
Tactic 2: Cast WoI (plane) about 1" off the ground, shatter it, and everyone in the room takes damage while they stand on the floor. Use fly to stay above it.
Tactic 2a: More generally, use the plane form and break it to make an easy trap that deals fairly nice damage, and is technically not visible (a sheet of frigid air).
Tactic 3: Cast WoI (hemisphere) on the cleric, use at-will teleport to get inside, kill him to your heart's content. Repeat on the mage, and so forth.
Tactic 3b: More generally, isolate characters from their allies, and kill them one by one.
Tactic 4: Cast WoI (hemisphere) around the osyluth itself, minimum radius to surround it. Inside, cast at-will invisibility, cast at-will teleport, and now attack from above, invisible.
Tactic 4b: More generally, use the hemisphere form to hide yourself to cast certain spells, or protect yourself. Readying an action to recast the wall every time it breaks is fairly easy.
Other tactics include:
Casting the plane form about 4' off the ground, so only Small PCs can fit underneath, and then doming them inside, alone.
Casting the hemisphere form in the air arguably drops a large mass of ice on people, since only the plane form is required to be anchored.
A combination of hemisphere and plane form would allow you to pin people to the plane in midair.
Any creature with at-will spells can be incredibly dangerous, and wall spells can easily force a party's destruction. Most of these tactics I came up with when a friend needed help running an osyluth. It's not hard to think of creative uses for walls.