You only need two spells in the first 20 or so levels: fireball and icicle. Keep upgrading them until both are maxed out. I received S rank in every level.
I just beat the game. Locations of the four ingredients: raisins (farm next to Darkville; they are called "currants" instead of raisins), candy (candy shop in Darkville), Dwarf sugar (shop in Dwarf town), flour (mill in Sleeping Town).
Alladinsane, the apple is in the forest near dwarf town. You have to fight a tree monster to obtain it. But first, you need to get Big Bad Wolf from Halloween town to join your party.
Once you learn what strategy to use, the game becomes surprisingly easy. Right now I'm at wave 112 with 4 aaa guns, a tank, two spec ops with bar machine guns, a medic, and an officer; all are at max lvl. That's pretty much strong enough to own everything.