Aim for as many headshots as possible to earn more coins. Farm the Wanted missions to earn extra money to use on upgrades.
Restore energy
Instead of waiting for your energy to refill over time, advance the time on your device by one hour. Afterwards, set the time back to its correct value.
Upgrades
When upgrading your weapon, following the recommendations and improve the suggested areas. Missions will have stat requirements for your weapon in order to qualify.
