On the Hard Line Pursuit level in Chinatown, enable the "Unlimited Ammo", "Unlimited Health", and "Maximum Fire Power" cheats to start with the Grenade Launcher. If you hit a Mr. Smith with three direct Grenade Launcher shots, he will die. You can then get his gun, which is the best pistol in the game, if you do not already have one with the "Maximum Firepower" cheat. After playing the Hard Line Pursuit level, continue through the last two Chinatown levels so you can use the Launcher during them. The last level on Chinatown is the best for hunting Smiths, because there are so many of them. Also try to enable the "Invisibility" cheat so that the Smiths will not annoy you while you are trying to kill them.