You have multiple ways: Masking all the models polygons except the surface you want to flatten, then with Transpose in Move mode, set it’s position to the root position of your transformation, then click and drag the opposite circle of transpose to do a non uniform scale: it will flatten all the unmasked points to the same planar position.
An alternative is to use the Clip Brushes (mainly Clip Curve, by pressing CTRL+Shift keys and selecting it as well as when using it) which will flatten all the points (and then polys) along the curve that you will draw with the curve.
I hope that helps!