Alright im gonna say whats best like in terms of facts not my opinions im talking FACTS.
Never waste money on a rocket launcher to kill a boss.
Dont upgrade any of your weapons at the start. Once you get the punisher sell the handgun but dont upgrade the punisher. you wait for the Red 9 and start upgrading cause thats the best pistol in the game cause of its power.
Shotguns: Dont upgrade any until you get the striker. You DO sell the shotgun to buy the riot gun but dont upgrade any.
Also you buy the Semi Auto Rifle when it becomes available and the scope and you keep upgrading that.
TMP is mostly for fun for people that want it but its not recomended. Dont buy any magnums cause you find a boken butterfly for free in the game