Hardest Monster you've ever faced?

[Surrealistik]

I don't see how water is like earth at all. While we're at it let's say moving through air is the same thing.

While we're talking about earth though, the ground will count as blocking terrain which means you don't have line of effect to it, so you can't place a zone there, anyways.

The zone's not underground; the origin square is on the surface. That said, Earth Roots' slide effect doesn't need line of effect to begin with.

But here's the real kicker: The Tarrasque just willingly ended its movement in the Web:

Move 1: Burrow 2 squares, top 2 squares of the Tarrasque are in the web, Tarrasque is immobilized, and cannot move again.

Say the Tarrasque saves, and had reserved an action to burrow again; another two squares. It's occupying every square that was previously beneath it. I don't even need to explicitly slide it through blocking terrain (because it occupies every square), and I have line of effect to the squares I want to slide it into. The medium/substance my forced movement is sliding it through clearly supports the Tarrasque. I think we're done here.