hm
1. Why not put the wall right in front of the dragon as it's moving? There is no save, so he couldn't prevent it from forming like the wall of ice.
2. Dragon will likely hit the wall, bounce, and crash to the ground (he doesn't have enough altitude to recover from a stall). If it has wingover, it could attempt a reflex save against the wall's DC (the wall doesn't allow a save, but it makes sense to add one in this context). If the dragon makes the save, it reacts quickly enough to perform a sudden wingover, otherwise it crashes into the wall.
3. This is the important part - if the wizard times it correctly, the dragon will fall close to the delaying fighters and clerics on the ground who will tear into it with melee attacks and touch spells.
If you think this is a good tactic against a dragon, try putting a wall of force in front of fast-moving ship. Considering the ship's mass, the wall will crack the hull like an egg. If you place the wall above the deck, it will snap masts, shred sails, and tear out the rigging. The ship would be dead in the water.
Wall of force is a pretty goofy spell when applied against large fast-moving objects, being unbreakable *and* invisible.