In campaigns based on the Normandy expansion, where both sides have a "hero" company that can "level up", I would use the following rules:
The winning side can advance squads with at least two surviving figures (MGs and mortars counting as one figure).
The losing side can advance squads with three or four surviving figures (MGs and mortars counting as one figure). They gain some experience from being in combat, even if they lose, after all.
In a draw, both sides can advance squads with three or four surviving figures (MGs and mortars counting as one figure).
What do you think?