Since you already keep your layers separated out, it should be easy - the trick with the low opacity is to do a lot of overlapping. the more you paint over the same spot the more opaque it will get, so you do that in the center areas of the region you want to look like, say, grassland. then let it fade out as it goes into the hills. Forests don't need the opacity but if you have a broken up edge, it should work.

The other thing you might want to try is blending modes - overlay for the forests, etc.