-) Beat a chapter using only one single weapon. Melee is allowed though and I think so are scripted turret parts. Do this with every weapon type, including Leon's and Chris' knives (they are different knives, so you will get two medals), Sherry's stun rod and Jake's bare hands.
-) Beat a chapter without going into dying status.
-) Break a crate by sliding into it.
-) Play Mercs. I don't know how many Mercs medals there are, though.
-) Beat a chapter that has a stealth section without getting spotted.
-) In the Leon and Chris story intersection, be faster than the other team (also nets you some treasures).
-) Get five headshots in a row.
-) Jake's campaign, chapter 3. Find all 16 ID medals.
There's more, of course. Just going off the top of my head.
"Oh, are they merely crying, father?" "Yes, true weeping is yet to come." - Nick Cave & The Bad Seeds