Just a little clear up and few extra tips to what ShinKazuki said:
1 - Save up and buy a gold booster pack, any of the gold characters will be strong enough to carry you through most of the game and make money much faster.
2 - Go to your character select screen and remove all characters from your team roster.
3 - Place your current gold card in slot 1, move it to slot 2, then slot 3 (leave it in slot 3) and back out.
4 - You will now have 3 of the character you placed in your team roster (3 Batmans, 3 Supermans etc)
5 - Every time you battle you will gain 3x experience for that character but also use 3x the energy.
6 - To replenish energy faster you can forward your world clock on your Ipod, Iphone, Ipad about an hour or 2.
Have fun!! Also try and replay the bonus missions for the best experience and money.