Here is the list of the twenty hidden trophies you can unlock that are not displayed on the trophies list in game until you unlock them.
Aerial Superiority – Kill 5 Helghast while in the air (Bronze) – Killed 5 Helghast while airborne using the Jetpack
Bring It Down – Defeat the MAWLR (Bronze) – Defeated the MAWLR defending the Space Elevator
Completist – Destroy everything on the MAWLR (Bronze) – Destroyed every destructible weapon on the MAWLR while on foot and on the Intruder
Double Trouble – Reach the river in Co-op (Bronze) – Made it to the Corinth River in Co-op mode
Excessive Force – Kill a lone Helghast with the WASP secondary fire (Bronze) – Used the secondary fire function of the WASP launcher to kill a single Helghast
Frazzle Dazzle – Kill 3 Helghast with one shot from the StA5X Arc Cannon (Bronze) – Used the StA5X Arc Cannon to kill 3 Helghast with one shot
Go Down And Stay Down – Destroy the ATAC (Bronze) – Defeated the ATAC outside the Stahl Arms facility
Iced – Destroy all Ice-Saws and Dropships (Bronze) – Destroyed all 4 Helghast Ice-Saws and all 6 Dropships
Into The Lair – Reach the cable car (Bronze) – Reached the cable car and gained access to Stahl Arms South
Iron Man – Get a kill with the Exo (Bronze) – Killed a player using the Exo in any multiplayer mode
ISA TV – Establish communications (Bronze) – Made contact with Earth
Jail Break – Liberate Narville (Bronze) – Liberated Narville from Stahl Arms South
Let’s Go Home – Destroy Stahl’s Cruiser (Gold) – Destroyed Stahl’s Cruiser and left the planet on any difficulty
Never There – Sneak past Helghast (Bronze) – Sneaked past all Helghast in the Jungle without alerting any of them
Pilot’s Wings – Mid-air kill (Bronze) – Got a mid-air kill using the Jet Pack in any multiplayer mode
Save The Intruders – Arc APCs destroyed (Bronze) – Successfully assisted with defeating the Arc APCs
Spread The Love – Kill 5 Helghast at once using the WASP’s secondary fire (Bronze) – Killed 5 Helghast at once using the secondary fire mode of the WASP launcher
Stranded Together – Reach the Extraction Point in Co-op (Bronze) – Made it to the Extraction Point in Co-op mode
Time For A Dip – Reach the river (Bronze) – Reached the Corinth River with the convoy
You Drive – Drive the Mobile Factory (Bronze) – Took over the controls of the Mobile Factory