Here is the full list of all 12 Flashback trophies - there are 7 bronze, 4 silver and 1 gold.
Complete the game on Expert
Without using Rewind on Expert
Without using Rewind on Easy/Normal
Complete the game on Normal
Complete the game on Easy
Complete level 4 (Earth)
Complete level 3 (Death Tower)
Kill 200 enemies
Complete level 2 (New Washington)
Kill 50 enemies
Complete level 1 (Jungle)
Kill an enemy indirectly