Switch leaders occasionally because they gain the most experience points from missions. Make sure that all characters stay even in their levels by farming the same missions repeatedly with different characters as leader.
Faster reloading
Tap the ammo icon at the top-right corner to reload, then swipe the screen after the meter starts to fill to speed the process.
Money
Grind Boss levels to earn more money. If possible, do so under the "Go Alone!" option to avoid deductions for having allies. If you have no money concerns, make sure you bring an ally on every mission to make them easier.
Scoring
Shoot moving enemies and distant enemies for more points. Build kill streaks to multiply your score. When you see an enemy highlighted in white, shoot them quickly for an extra bonus. Always follow the route with more enemies. Headshots result in more points.
Targeting
For better scoring and aiming, shoot by tapping the bottom of the screen instead of tapping on the enemies. A bottom tap results in an instant hit, which eliminating the chance of error with moving or distant targets.
Various strategies
Your character automatically takes cover. Tap to have him leave cover to shoot. If enemies throw grenades, you can shoot them out of the air before they explode. Civilians are highlighted in green. Killing a civilian will end a combo streak and reduce your money. Some missions will fail if too many civilians are killed. Enemy targets are highlighted in red. Special enemies will momentarily have a white aura that turns red. The special enemies allow you to build up a combo meter that will drop if fail to kill them before they escape. Get combos to gain stars and other rewards. Aim for the head for faster kills and to keep combos running.
Weapons
Due to the cost of purchasing a new weapon, it may be more economical to upgrade a current weapon. Some missions require you to use specific weapons. Do not sell all your extra weapons; try to keep at least three of each type at all times for each crew member. Rare and epic weapons are the best available because they do the most damage. However, they are difficult to optain. You can get them for free during the gun rewards after missions are completed. They can also be purchased from the shop when you have enough money. You can increase the rarity of a weapon by leveling it up.
Experience...