A few tips for playing GTA 5 as a solo player are to take cover and to stay out of the open as much as possible. Press the appropriate button next to an object to duck out of the way.
Another tip is to kill enemies with a shot to the head to avoid a lengthy gun fight. Also, buy an airport hangar to avoid increasing the character's wanted level when entering most airport or military installations in the game. Remember that side activities are also beneficial to the character's advancement. Doing yoga with Michael, for instance, increases the player's special ability skill.
Maxing out available weapons at Ammu-Nation is one other tip. Level these up to improve accuracy, as well as firing and bullet capabilities. Also, avoid costly vehicle repair bills, by controlling the vehicle while it is in midair. Use the left analogue stick to position the vehicle to land as safely as possible.