Just an idea...
Lay down something temporarily, like a road or track, along the length of where you'd like your slope.
Use the linage-tools thingy that allow you to grab the yellow anchor-point on it (for adjusting heights on roads, tracks, fences, etc..), then slightly/gradually raise the road/track on on end, approximating where you'd like the slope.
Use the paint-brush tools, enlarge the size of the paint-brush and another other options (such as falloff, etc..) with it accordingly, then use the terrain-snap-to option, and then run the paint-brush along the length of the road/track.
This should raise the terrain up-to the level of the road/track accordingly for that area.
When done, simply delete the road/track if you no longer want it there. Profit.
Well that's one way I might do it. Excuse if some of my terminology for the tools options I used in my explanation are a bit off, but I think you'll get the idea of what I mean.