Only Good Guys Win Medals Achievement in Resident Evil: Revelations 2

There are a total of 20 medals to obtain in Episode 1. Dying resets the numbers back to your last checkpoint, and does not stack between multiple playthroughs. You can view your progress of medals from the main menu and during gameplay as well. The ones you need will be darker and not highlighted, which can be somewhat difficult to notice. 

Claire/Moira
Gold: Knocked down an enemy 5 times by blinding them with light, then hitting with physical attack.
Solution: Use Moira to stun enemies with flashlight, then quickly switch to Claire and use  when prompted to knock them down.
Gold: Defeated 8 enemies with rotating bladed
Solution: Run past most enemies until you the blades going, then lure them in. After putting the cog key in place, do not turn on the fire trap. Have Moira start using her crowbar after you clear the enemies, and it will keep spawning enemies 2 at a time. Blind them with Moira and try to kick them into the spinning blades.
Silver: Stunned 10 enemies with Moira's light
Solution: Just make sure you stun enough enemies. Can utilize unlimited enemy spawn at end mentioned above.
Silver: Defeated 8 enemies with sub-weapons
Solution: These weapons are the bottles you can throw that blow up, catch fire, etc. Saving them for big groups of enemies can make this easy. If you wish, you can play multiple times saving up bottles.
Silver: Defeated 3 enemies using the stealth attack
Solution: There will be a section that introduces you to stealth. You can use it here and a little bit later in the holding cell area. In later episodes you gain a Smoke bottle, which lets you stealth enemies easily even if they are after you. 
Silver: Defeated 8 enemies with a headshot
Solution: This should be obtained naturally. You may utilize the unlimited enemies mentioned above.
Bronze: Found 3 treasures
Solution: Treasures are the gems that give you BP (Bounty Points). You should be able to find these while going for hidden items with Moira and her military boxes.
Bronze: Killed 10 Giant Whip Spiders
Solution: The spiders are in the compactor area. There are tons of them, you can't miss this spot. Kill them all.
Bronze: Opened 3 military boxes
Solution: These are the dark boxes you can use Moira to open. They should be easy enough to find on your own, or look up a video online.
Bronze: Found 6 hidden items with Moira
Solution: You will be taught how to use your flashlight to point these out. Use Moira mostly and you will find a lot more than 6 of them around.

Barry/Natalia
Gold: Defeated 5 Revenant by only hitting their weak spots
Solution: These are the disfigured enemies that have those yellow sacks you can expose and shoot to kill them fast. Switch to Natalia to see where this spot is, then switch back and attack that spot with Barry. Exposing the weak spot is a must and doesn't hurt the progress. At the end of the chapter, once you knock down the doors with the crane, you may stay and fight enemies. They are endless.
Gold: Didn't fire a gun in the detention center control area
Solution: You will get this while only using the knife. 
Silver: Hit enemies with Natalia's brick 15 times
Solution: Using Natalia, throw bricks to stun enemies. They are highlighted in a light blue around the environment. You can stun the same enemy, but the bricks do break.
Silver: Defeated 2 enemies with a follow-up attack
Solution: After stunning an enemy with Natalia, use Barry to finish the enemy (unlocked perk Follow-Up in skills first). 
Silver: Defeated 8 enemies with sub-weapons
Solution: There will be multiple areas that will have 2+ enemies you can kill with bottles. What you can do is just save your bottles and use them when you feel things will work out best for you (like having 4 explosive and 4 fire for example). 
Silver: Defeated 6 enemies using the stealth attack
Solution: Barry has multiple enemies he can sneak up on to stealth kill. In other episodes you will be able to make a Smoke bottle that you can throw to confuse enemies. You can stealth kill them then if you are having a hard time.
Bronze: Defeated 6 enemies with the knife
Solution: You will get this is in your knife run (and possible if you kill all the spiders in the start)
Bronze: Opened 2 gimmick boxes
Solution: These are the golden boxes you can use Natalia to open. There are more than 2 in the level and you will find one in the room above the cells and one in the forest.
Bronze: Found 6 hidden items with Natalia
Solution: Natalia can point at shiny objects to make them appear. Use her when you can to look around.
Bronze: Escaped the darkness of the forest, starting from the detour hut within 40 seconds
Solution: This invisible timer starts after you open the door of the hut that had the first revenant enemy. Run straight for the exit, avoiding the enemies. It ends with the bronze doors you open.