Cleaning every surface that could be exposed to the odor is the first and key step. This includes carpets, walls, ceilings and any furnishings in the apartment. Don't forget air filters should also be changed.
After the cleaning, I have found the Vaportek 3X industrial cartridge to work very well. It has the advantage over ozone in performance plus you won't have to worry about occupied or unoccupied, potential harm to anything in the apartment that might be caused by ozone.
Place a cartridge near the air return and keep the fan on. This will help diffuse the vapor throughout the apartment. Run over night and then air out the place.