When low on health, pause the game and go to the Unlockables section then select costumes and choose one. When your costume is changed your health will be replenished.
When low on health simply find a Race or Combat Tour. Once you've found one begin the mission and when it starts you will have full health, now pause the game and select Exit Mission you will now be out of the mission with full health.
To raise the City Event Difficulty you must do the following: