This is an AMAZING game and a lot of it comes down to practicing with the cars and weapons. And the music!!!
You can't just know what the strategy is - you have to be able to implement it. Don't race through the game from mission to mission. Take your time doing side missions, exploring, practicing. Run a LOT to build up your stamina. It really helps! The better you know each town, and know how to drive from place to place, the easier the missions are.