Do you hate to wait until your Energy is restored? Well I got Something for that!
Step 1 : Go to your date/time settings and change them to 1 hour later.
Step 2 : Go back to contract killer and there ya Go your Energy should be restored! (Full).
More Money Quicker Hack
Go to the settings and change the time to 11:59 pm and wait till it turns to 12:00 am. This helps because you usually get a bonus every day that you play and by doing this it helps you get money quicker.
Open the game and watch the video on the left bottom corner of the game map screen you receive a free credit, press home button to get out of game to the main menu and double press home button again and long click on the game icon till it shakes and shows minimize icon on it, close the game and open from the main menu again and watch the video again so you can get a credit again do as many times get as many credits.
Learn The Mission Locations
When you're playing, take note that the game only has a few different locations for missions. Learn them, and go for the easier locations first, so that you can win more XP much faster. You can unlock more locations as you go through the game.
Missions cost energy and when your energy is gone, you only have a couple choices. You can either buy more with credits, IAP, or just wait till they replenish over time (and lets face it - we HATE waiting!). They also fully replenish when you level up, thus if you're good at the game and can get bonus XP you can get a lot farther faster.
Early Game Play Hints
Early on in the game, the contracts are pretty easy. But, there is always the challenge to get a faster kill to get more XP. Go for those.