Draw the wall full height. Then cut the upper part with using SEO (solid element operations) with an object with the desired curvature as operator, then you hide its layer.
Do a search in this forum, the question is common. Also, this tip might help (the part about roads in terrain applies):
http://www.aecbytes.com/tipsandtricks/2006/issue10-archicad.html
Just add hiding of the mesh's layer in the end.
AC4.1-AC26SWE; MacOS13.5.1; MP5,1+MBP16,1