Never switch off the Aim Assist. It is a life-saving feature automatically that causes your gun to snap to the nearest target, giving you a much greater chance of scoring headshots during the single-player mode.
Keep in mind you can add Boosts, Upgrades and Attachments at any point during single-player with your well earned Blue Credits. Pause the game, click Shop button on the bottom of the screen and do your upgrade.
Spend Gold Credits to enhance multiplayer experience.