To reach Five-Star General rank, you need 46070 RP. By playing through every mission in the game you will be in the 10-15k range depending on how many missions you repeat. The fastest way to gain RP is under the Colonel mission "Dead Alive", with this setup I finish the mission in 22-28 seconds:
Receiver: Spreader
Barrel: OCP-90
Magazine: Karabiner
Sight: Heavenly Spear
Stock: OCP-90
Ammo: Hollow-Point
Mods: Magazine, Speed Loader
Proficiencies: Speed Loader
Completing the mission nets you ~500 RP and $8800, so this will require many replays. Once you hit 46069 RP you will stop gaining RP until you defeat the final boss again, after which the trophy will unlock.