Complete the game once, then use as much time as desired to get the remaining green gels for all the upgrades. Also salvage parts and ammunition. When your ammo and skills are ready, start a speed run in New Game + mode from Chapter 1 under the casual difficulty setting. Starting a New Game+ from any other chapter will retain your previously recorded time, but starting from Chapter 1 will reset all subsequent times. Once started, play as fast as possible. Keep in mind that cinematics will add to your time; skip them whenever possible. Skip collecting gel, and get ammunition only when it is in plain sight. Go to the safe room only required. Save the rocket launcher and Magnum for Bosses, along with incapacitating bolts.