I had a similar situation with a vacation house that was built into the
side of a hill. The concrete wall was only on the hill side, and the
rest was framed in - acutally used 2x3, as there was no load on this
wall. I first put up about 1" bead board as insulation, then framed up
against that. I simply anchored the sill with concrete nails (just to
keep it from kicking out). At the top, I hooked into the rafters. It
was actually "free standing," and there was no flex. I then put dry
wall over that, and then paneling. Wiring of course went in before the
dry wall/paneling. The span might have been around 15', if I recall
correctly.

Others may have their opinions.