Smooth Localized Mesh

Hey guys,

I'm trying to smooth a joined mesh on a specific side of their faces.

This is what I have,

Referring to this project by Iker Mugarra Flores [http://www.grasshopper3d.com/m/photo?id=2985220:Photo:1187626&c...]

and this tutorial by ModeLab [http://grasshopperprimer.com/en/1-foundations/1-6/4_Mesh%20Operatio...]

I understand that i need to remove some of the face so the naked edges will be the anchor for the non-smooth area. I'm just not sure how to cull the external faces specifically. Any idea??

Thanks in advance. 

Cheers