You can earn lots of coins from Endless mode and farming previously completed levels.
Defense
Pay attention to where attacks are coming from. Watch for a red glow to either your left or right. Immediately shift to that direction to quickly eliminate the threat.
Diamonds
Diamonds are the game's premium currency. Redemption codes for diamonds are posted regularly on the game's Facebook and Twitter feeds.
Weapons
Use weapons based on your preferences and situation. The pistol is good at short range and does moderate damage. The machine gun has better range and is capable of rapid fire, but does less damage than the pistol. Sniper rifles and rocket launchers fire slowly but do a lot damage.