Sharpshooter doesn't get around full cover and magically allow your PCs to see through objects. Gaining full cover is not difficult unless you are a huge creature. And it's not metagaming to have my monsters do things that are basic common sense and survival instinct. I wouldn't expect the PCs to stand in the open taking arrows in the face from 600ft, why would I expect the monsters to?
If the monsters are hiding behind something like the trunk of a large tree, a large bush, or even tall grass large enough to cover them, they are fully covered. If the PC is flying and the monsters are hiding under the canopy of trees, in a building, under a cart, anything where the PCs cannot see them, they are fully covered. If your monsters jump in a trench that is deep enough to break line of sight to the PC, they are fully covered. Same with stepping around the corner or into a building, ducking behind a large cart or rock or anything that is big enough to break line of sight. having things in the environment that are large enough for a monster to hide behind is pretty simple really.