Severe ARDS can be defined in various ways, usually by the lung injury score. In general it means you can't ventilate the patient with reasonable airway pressures and fractions of inspired oxygen.
Rescue therapies are modalities considered unproven in that they haven't been validated by high level evidence. As measures of last resort for the patient who cannot be managed by other means, however, they may offer the only hope for survival. Rescue therapies were reviewed in a recent issue of Critical Care Medicine and include recruitment maneuvers, high PEEP, high frequency oscillation, inhaled nitric oxide, prone positioning, corticosteroids and extracorporeal life support.