This is not a full guide but just some advices that you will need for the gold medals:
- There is a time bonus: It means faster you complete the level, more points you will get at the end.
- Always go for the headshots: Headshots will grant much-much more points than regular kills. If you done the level fast enough but couldn't get gold medal, that's because you need more headshots.
- Shoot the vials: Throughout the levels you will see some vials on the ground. Shooting them give you extra points.
- Keep shooting finished enemies: It takes a while until a zombie completely dies, you can keep shooting them for more points.
Finally, the trophy is much easier with a coop partner. Good luck! :)