You earn Medals at the end of each mission based on various criteria. The mission assessment screen is broken down into 3 sections which from left to right are Challenge Bonus, Kill Bonus & Style Bonus
These scores are then multiplied by a modifier based on the difficulty level.
Cadet 1.00 x
Officer 1.25 x
Detective 1.50 x
All golds can be achieved on cadet I can verify this myself, but you do have to max out your score. Whilst the exact scores for gold are unknown a good rule of thumb is if you score between 9k & 10k you'll get a gold although there are some exceptions such as mission 6 which is definitely more than 9500pts as this only gives a silver.)
Bear in mind that you may need less points on higher levels due to the modifiers but those points are a lot harder to earn, you will also be slower and more likely to die.
Clear Time Bonus Up to 3000pts (Complete the level fast as possible)
Kill Variation Up to 1500pts (Mix up Gun kills, Shadow kills & Melee Kills)
Kill Bonuses (Up to 500pts each)
Style Bonuses* (All Up to 1000pts **except takedowns which I could only get a max 500pts)
Survival (To max out neither you or shadow must die).
Counters (When in melee combat counter with X )
Takedowns (Hit A when take down prompt appears YYY combo is good to prompt a take down)
Stealth Kills (Only possible in the shadow sections)
Tag Team ( Either shoot or punch a enemy then order shadow to take him down)
* I made sure I got at least 10 of each
** If anyone can confirm you can get 1000pts for takedowns let me know how many it takes as i always got 500pts