Instructions
Reach the final battle between Megatron and Optimus Prime. In order to unlock all Decepticons for play, you will need to complete the game and beat Megatron.
Block Megatron's attacks. Pick up a tree using the circle button and throw it at him to knock him out of attack mode. Press the square button to counter with melee attacks until he flies away. When he bombs the area, block, then throw another tree at him. Attack and repeat until you defeat him.
Replay any mission. You can now select any of the Decepticons for your character. This also works in multiplayer mode.