The sneak up trophy i did by doing the boat scene again and again. you can usually get about 2 or 3 sneak attacks on the boat if you do it correctly. also, after the third wave of enemies, restart the checkpoint, and keep doing it until you have the trophy.
another place is in chapter 4 i think, just before the fortress, and there two enemies with their backs to you before you enter that particular scene. Kill the two enemies by sneaking up to them, and then die to restart the checkpoint and keep doing it.