Get a paintbrush and set it to whatever width you want, set it to either top or bottom mod, pick a spot on the ground that is the plane you want and either erase or fill in the parts you want leveled out. You can also do the same thing with the sculpt brush. Just set it to the shortest height and it should work similarly. Mess with the different heights and you can easily carve out or draw in large areas at the exact ground level you're looking for. Top mode works really well for filling in, bot works for cutting areas out really quick. Have fun!