Something you need to be aware of is that the Grasshopper-ARCHICAD connection is a relatively new development so it does not offer all the capabilities of what you can do in ARCHICAD, including element Connection commands.
One possible solution may be to create a Complex Wall with the exact section profile you need (slanted top) and then use that Wall type in Grasshopper when generating the Wall.
Otherwise you would have to trim the Wall with the Roof in ARCHICAD.
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