Mike Mearls’ unofficial ruling is that Wall of Force does block spells, including lines of effect
Quoting Mike Mearls on Twitter:
Aug 28Jim Miller @pokereleran@mikemearls Is there a line of effect in D&D and does Wall of Force block it?
Mike Mearls – @mikemearls@pokereleran in general, a barrier that stops physical objects stops spells
Rulings from developers on Twitter are in no way considered official rulings, but this is still a ruling from someone who knows the game supremely well.
The Tarrasque would be pushed to one side of the wall when you cast the spell:
If the wall cuts through a creature’s space when it appears, the creature is pushed to one side of the wall (your choice which side).
If you cast the wall inside the beast, it clearly "cuts through a creature’s space when it appears", and therefore "the creature is pushed to one side of the wall".
Of course, at that point it wouldn't hinder the creature's movement much.
Regarding how this should be narrated by the DM, there isn't much to go on in the spell's description.
For example, what if the wizard shapes the wall into a sphere around himself? This is such an unusual situation, so the DM would have quite a bit of latitude in determining how the result is narrated.
It is fun to imagine that the wizard, in a giant bile filled snow globe, is unceremoniously ejected from the Tarrasque like a hairball from a cat. I'm not sure who comes out ahead in that scenario.
Best Answer
No
The text of greater dispel magic included in your link states:
There is no mention wall of force in this exception clause, and wall of force is not removed by remove curse. Therefore if a wall of force is either targeted directly or included in the area, the spell functions as dispel magic against it. According to the wall description, the wall is not dispelled by this.
Note that the wall of force will also block line of effect, and so anything on the other side of the wall is not subject to dispel even if it would otherwise be within the dispel area.