A lance of heavies will be facing nearly a battalion of mixed infantry with field guns/artillery, Procas, and VTOLs. Proper tactics and field maneuvering will easily allow the lance to be overwhelmed and defeated. If you decide to maneuver 14 to 16 Procas on an open plain against a lance of heavy traditional BattleMechs, they would still be able to cause some damage but would probably be destroyed - at which point, the commanding officer should be arrested for dereliction of duty.

That said, you don't occupy with a lance of heavies and you certainly don't defend by waltzing out into an open field and exchange shots with something double your weight. Utilize terrain, utilize artillery, focus on guarding infantry and letting the big guns do the work.