Buy the energy drink from the food shop. This is a very important upgrade because it will add one attack per turn to your battles. So the first energy drink will upgrade you to five attacks instead of four, and the third energy drink will upgrade you to six attacks instead of five.
After you buy everything that there is to buy in one of the shop buildings (or when you buy the energy drink in the case of the food shop), a construction worker will appear outside of the building. Tap him, and he’ll ask for building materials again. Give him what he needs and he will upgrade the shop; then you’ll be able to go back in and find a new selection of goods to purchase.
Buy the weapons and armor upgrade immediately from the equipment shop (the one in the top left corner), as soon as you can afford them. They will add 3 attack and 3 defense, respectively, to your character’s stats; however, the reason you should add them immediately is because they level up as they are used. Go to your character’s house to check the level up stats and how far along the upgrade bar is. With each level gained, you’ll gain 3 more attack/defense depending on the piece of equipment that leveled up.
When you have the extra attack due to buying an energy drink, you will be able to use more attacks either before or after the combo attack. If you hit after a combo, you’ll juggle the enemy up in the air and earn bonus coins. If you have 8+ attacks per turn, then you will be able to unleash two combos per turn. 9+ and you will be able to do one 5-hit combo (spell combo) and one standard combo. 10+ and you will be able to do two spell combos.
The guy at the top of the top-left store will sell you building parts in exchange for other building parts. Honestly, this is probably the most useless store in the game unless you need a specific part in small quantities in order to complete a building. For example, if you need 5 copper wire but you have plenty of bars, go trade the bars for copper wire to finish the building.
Watch out for the gold enemy to pop up randomly in the non-endless levels. They shimmer and they have very strong attacks, but almost no defense. Beat them up and take extra care to defend with perfect accuracy. When you beat them, they will give you 6x the amount of coins as a standard enemy will.