You must win three consecutive games of rock paper scissors to unlock this achievement. To start a game, press Y while not in combat next to your partner. Use the D-pad to select rock, paper, or scissors.
To start the game with fully upgraded weapons, start a new game and use the starting money to upgrade our buy new weapons. When you reach the cutscene with the moral choice, select the moral choice, then go to the main menu during the custscene with the explosions. Select Campaign mode, and and select the one that was just played. Select "New" and you will now have all previously purchased weapons, upgrades, and money from the previous mission. You will be able to upgrade your weapons again. Repeat until you have fully upgraded and purchased all starting weapons.