Medals are awarded based on how well you held your ground and performed during the mission (i.e. how well you protected the objective or how quickly you defeated a boss). The highest-ranking Gold Medals are earned if no less than 80% of the primary objective's health remains by the end.
For boss levels, defeat the following within the corresponding estimated time frame:
LEVEL - TIME FRAME Africa - 135 seconds. Europe - 40 seconds. Pacific - 320 seconds.