Just update the core items to maximum before buying any other items begining with what is more suitable(if the are more power damage champions you fight against you update the item with power armor faster and viceversa) .The Radion Shard is so you can build power damage fast.You upgrade it to maximum and then sell it to buy the more expansive items(radion shard restores all coins spent on it) and then repeat the process untill your item bar si full.After you sell a stack of radion shard,chose from the optional items 3 that you think will help you the most and give you the upper hand(personally i would chose first The Starheart and then Ra's al Ghul Mantle and the sixth depending on the situation).This combo gave me a  very hard to kill  and aggressive Nightmare Superman and i hope it helps you too. 



P.S.: The first 3 starter items are for Gotham Hights. The other 2 are for Gotham Devided since this guide works for both.


P.P.S.: When you pick the stolen powers i'd recomend nightmare superman's teleport for gotham devided and doomsday invulnerability for gotham hights.Ivy heal wave is necessary for both.