Place your device on a flat surface and use both hands for control. Use one hand to drag and move your character on the screen from left to right to avoid projectiles and use your other hand to aim. Although you get a bonus for destroying all enemies, sometimes swiping over them all is not that best strategy. Tap them one at a time if they are moving fast or you do not have time to wait for all of them to appear.
Restore health
Advance the time on your device to revive all your gunfighters. Then, set the time back to its correct value.
Upgrades
Upgrade hearts and ammunition first to improve your survivability. Although it may be better to have one character when first starting, try to balance all your characters as you progress.
Weapons
Use weapons to their capabilities. For example use the shotgun to clear enemies in tight clusters; use the machine gun with the Stunlock ability in later levels.