After reaching stage 15, begin investing in companions. Start with the more expensive ones, which do more damage. Upgrade all of them several times. Although their base damage is not useful at the start, if you keep upgrading companions you will eventually reach a point where special skills for them will become available. This will increase their base damage or provide other bonuses.
Easy coins
Use the following trick after getting a set of powerful companions. Disable the auto screen lock feature on your device and make sure it is charging. Allow the game to run on its own, and your companions will eventually kill all monsters that spawn except for Bosses. However after getting defeated by their first Boss, they will only get regular monsters spawning from then on. You will get lots of coins by letting the game to simply remain this way.
Invisible Player
In the Character Customization screen, tap on Change Name. Enter the following code.
Code
Effect
Clear
Cancel the code (Player becomes visible again)
Ninja
Invisible Player.
Special powers
Get to Boss battles as soon as possible. Leave it if you cannot win, then return later when you have super skills ready or have just tapped a fairy and have a bonus. Both of those factors will make Boss battles much easier.
Sword Master Names
Click on your main hero's icon, and rename your hero to these following noises. A sound will play when you enter them.
Effect
Code
Clears all name codes
Clear
Gives snow weather effect (may be hard to see)
Frozen
Make farting noises when you tap
Poop
Player becomes invisible
Ninja
Replace gold coins with silver coins
Silver
Replace tap damage with "ERROR"
Cheater
Replace tap damage with 1's and 0's
Robot
Titans become small (except Chesterson)
Mini
Unknown effect
Link
Unknown effect
Zelda
Upgrades
Invest in Tap Damage first, and increase it as much as possible. Ignore all other upgrades until stage 15. This will be your main source of damage for most of the early part of the game. You will need to save a lot of money before beginning to invest in Tap Damage. After a few upgrades, you will get an option to invest a larger amount for an extra bonus. Always select that option to maximize your Tap Damage.
Companions...