So, you are talking about wheat, but really, what you are concerned about is agriculture. It doesn't matter if you are growing wheat, rice, barley, turnips, or you have sheep grazing, agriculture need lots of land.
Agriculture cannot co-exists with large quantities of rampaging monsters. You are completely right about this. This means that there has to be "civilized" areas where agriculture is possible. Areas with lots of monsters simply don't have a lot of humans living there. It's "the wilderness".
The best solution is to have "wild" areas and "agricultural" areas. I will note that there is a LOT of bad world design out there.