Here is the full list of all 13 Demon Pit trophies - there are 1 silver, 11 gold and 1 platinum.
Airborne for 60 seconds
Completionist
Reached wave 10
Reached wave 20
Reached wave 30
Reached wave 40
Reached wave 50
Reached wave 60
Killed 666 enemies
Collected all weapons
Killed 13 Skull Mech
Killed 13 Hammerheads
Killed 6 Enemies in Single explosion