Headlines
Progress further into the game by focussing on completing the 'headlines' (quests) in each of the three levels. Doing this will also allow you to max out the rewards which will always include diamonds.
Always Upgrade
The key to success in this game is to make sure you upgrade whenever possible as doing so gives you more protection from attack, ups your gang level and also Unlocks new content.
Time Upgrades
Time upgrades or new powers to give to your super-villains for the end of your playthough so it does no interfere with Gameplay and it will be ready for when you next come back. Doing this will also help stop you spending gems on finishing upgrading quickly.
Super-Villains
Try to avoid the temptation of just using one super-villain and instead try to use them all as doing so will give them boosts which you can take advantage of if you ever need to utilise their special moves.
Henchmen
Add luxury items like furniture to your hideout so it can be used as cover in the event of a raid by your henchmen. Doing so will also generate cash and Respect Points which can then be used to purchase better thugs and other perks.
Attacking
Always place your super-villain at the front of an attack as they have way more health than your henchman and will be able to soak up more hits. If your super-villain starts to take too much damage you can always move them.
Special Moves
Work out how long your special moves cooldown is and try to make sure they are available for when you really need them at the end of challenges where the fights tend to be tougher.
Diamond Safes
Always target and smash open the diamond safes that are available in some of the levels as the currency inside can be used to speed things up at your hideout.