you can make a straight wall with the terrain raise. Just have to click carefully and precisely. Its not easy, and takes practice, but its doable.
- you can't go all the way up and have a perfectly level wall. The wall can only be as high as the lowest ground segment can be raised if you want level.
-if you misclick, it will get crooked and janky in a hurry. lay out wood beams or floor tiles or something and trace along that.
-if you hoe the top to level it, the whole wall will change a little. It quickly becomes a blob instead of a wall.
- you can't easily fix mistakes, you have to cut it all the way out and do over.

at some point its best to just say good enough. There will always be a few imperfections in a large terrain box.