Near the beginning of the level there are 4 Mutation Stations, one of which you need to destroy to continue the level. Keep killing zombies until the the third bar is about a third to half full or until you have about 30 seconds left on the clock, whichever comes first, then destroy the remaining vials and end the level. Around this star are 3 segmented bars that fill in as you score. On the left side of the screen is your score with the star designation above it.

Allow zombies to spawn and kill them with headshots whenever possible. Run through as fast as you can and destroy all Mutation Stations except leave a few vials in the final one. These can be done solo, but a lot of stuff is actually easier with a co-op buddy, so I recommend playing them co-op. I'm assuming that you have at least a passing familiarity with the levels so I won't be giving detailed walkthroughs, merely highlighting spots you'll want to go through. Okay, so this will not be a comprehensive step-by-step solution to get gold, but more of a series of tips that will help achieve the gold score for each level.

Far cry 4 zombies