I am trying to model a cat head, and I need it slightly elongated in the back. I don’t know how to describe it any better than that. I just figured out the flattening, I just need a huge sculpting brush.
In edit mode, to the right of your vertex/edge/face select options. There is a little gray circle (proportional editing.) Enable that and play with the faloff options (smooth, sharp, root, etc.) When you select an item in edit mode and move it, nearby verticies will also move. Use your mousewheel while moving object to control the area of effect.
do the proportional edit thing for the back, then to flatten select the loops and press s y 0.
that presupposes that your flattening the front, in front view not some weird awkward angle
Edit mode, Select the edge you want to move G G Y then move the mouse.
A better option could be to select the edge shift-D to duplicate, move that into position, adjust the shape if needed and bridge the edges (W, E.)