If you follow the steps below you will be able to do this. I was unaware of the wall type and building materials you are using so I just used a standard Brick Veneer with Austral Bricks.
1. The first thing I do when doing unusual walls or models is to set up my drawing with just basic line and text. Check below.
2. Is to setup your wall with heights and materials based off your preperation.
3. Draw your wall and make sure your wall matches up with your preparation drawing.
4. Now you want to go inside the component/group and select the face you need to push-pull. Once the face is selected (blue dots) you then can draw your line that you would like to cut so that the wall becomes slopped.
5. You then want to grab SketchUps Push-Pull tool. With the push Pull Tool you want to push it back the same distance as the thickness of that wall. See Below.
6. You now want to do this for your insulation or other wall.
7. You will now be finished. Make sure that it matches with your preparation. Once done you can then delete your preparation.