If you have choices, you better play the game with iPhone considering the easier control as well as making a good aim and headshot. Just like with other games on iOS, Android and Facebook, energy refills automatically over time and what you need to do is to either wait for it or spend premium currency to instantly regenerate your energy. It should ignite your interest (They gave me the number of problems). As for iPad since this is bigger, it will makes thing more complicated. It usually happens right after I was striving for Sniper 3D Assassin Cheats. Grind on Wanted missions.
http://maddenmobilehack.net/sniper-3d-assassin-cheats-hack-tools/