TIPS: (1) Dump a little money into your character, but most of it straight into your spreader gun. Ignore the other guns. (2) Don't go crazy looking for hidden areas. In Chapter 7 you get an X-Ray that helps you find them easier. Spend your time getting there first (3) Only grind for diamonds if you can't get to the next chapter without upgrades. Once you get to Chapter 7, you can collect diamonds much faster than anywhere else.
Awesome megamega tower (18k Damage, 800 Range, 1 Reload) formula = 4 Sniper towers, 8 Damage boost, 4 Fire Rate boost. But, when on a level with no ghosts, a similar Area Burst megamega tower kicks butt.
 "Diamond Hollow II"){kind=icon}
 "Homerun In Berzerk Land: Berzerk Ball"){kind=icon}
 "Bubble Tanks Tower Defense"){kind=icon}
 "MegaDrill"){kind=icon}