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.
Move the grenade button to avoid accidental throws.
You don't have to keep swapping guns and grenades to one loadout in MP. In fact, you can purchase five more for 10,000, 20,000, 40,000, 80,000 and 160,000 Gold Credits each.