If you mean something like this, it's pretty easy. If the wall joins a slanted wall, it will do the join automatically as shown. If not, as shown on the other side, I am using a slanted wall (just like the opp. side) on a different layer (Solid Element Operator) and do a "Subtraction w/ upwards extrusion". I left the layer on in wireframe mode so you can see the operator.
ArchiCAD 25 & 24 USA
Windows 10 x64
Since ArchiCAD 9