Load the "Rats" mission from the Crime.net map. The "Rats" mission will become available once you reach Level 13. Play through the mission until you reach the "Bus Stop" side mission. The "Bus Stop" side mission requires you to kill all of Mendoza's men on the bus. It can be completed in 37 seconds. Kill all the men on the bus, then run back to the helicopter. The mission will end, and you will be rewarded with experience and money. When it says "Payday", go to the next screen to display three cards. Pick one of the cards to get a random item. When the item is shown, go to the menu screen, then select the "Restart Game" option. This will reload the "Bus Stop" side mission, but you will still keep the experience, money, and item you earned. You can keep farming experience, money, weapons, and mods by completing the "Bus Stop" side mission as many times as desired. Once you reach Level 50, go back to the Crime.net map, and look for the "Rats" mission with three yellow stars. Select the mission with the yellow stars, then keep completing the "Bus Stop" side mission until you reach Level 100.