Play a game under the normal difficulty setting using no first aid sprays and end with a time less than 1:30:00 with good shooting accuracy to get an S rank.
Save the game at the end. Load the saved game to have a rocket launcher with unlimited ammunition.
Defeating Neo Tyrant:
If Neo Tyrant is far away enough, unload on him with the handgun shoots and reloads very fast. Never use any other guns.