The best time to level up the weapons is Los Santos, you dont get really high wanted level,and the police isnt anything you cant handle. Ok to start equip the weapon that you want to level up (get to hitman level) and go to the street acroos the gym (in los santos) and get onto the roof of that gym. Now with whatever weapon you have, MANUALLY aim and fire at the cars, this will make them drive off, but you will get a few shots in, and it will count. If someone gets out of the car just keep shooting the car until it blows up. When the police come just shoot them, and kil them and shoot their car. When you are slightly low on health, or have around 3 stars, or your running out of Ammo, you can quickly jump of, and head straight for your hideout (hi-jack a car if you want). I found this a good place, however you can also do this from the room of your hideout (in ganton), to get up their, climb the wall next to the house and use that to climb onto the roof, however not as many cars will be coming, but this time you can just jump down and the save point will be there.