KUNAI is quite fair in terms of in-game currency. You can get coins by opening chests or defeating enemies and, while chests are limited, new enemies regularly respawn in every visited area. Since power-ups and upgrades do not cost an exaggerated amount of coins, you should be able to unlock most of them just by playing/completing the game, however I'll explain an easy method to speed up things, in case you've already opened all the chests and you don't want to wait too much for the enemies to respawn.

This method requires a few conditions to work properly (mainly the first 3 on the list):

• the Artificial Desert area to be available;
  
• the Coin Magnet upgrade to be purchased;
  
• the Rocket Launcher weapon to be unlocked;
  
• the Dash ability to be unlocked.

Below I explain the reasons behind each requirement:

• Based on my playthrough, each area of the game is internally divided into a few subareas (or group of rooms). The enemy respawn seems to be related to this internal subarea system: every time Tabby moves from one subarea to another, all the enemies respawn in the subarea just left. I noted that the Artificial Desert had a higher respawn rate than other areas in the game, probably because of smaller subareas, and one specific room allows you to make enemies respawn just by exiting and re-entering it.
  
• The Coin Magnet is one of the first upgrades you can purchase on the Tabb Store, it passively sucks coins to Tabby and makes picking them up faster.
  
• The Rocket Launcher is unlocked later in the game (after beating the boss of the Zen Mountain area), it can destroy multiple enemies in one shot and also makes them drop more coins.
  
• Lastly, the Dash ability is unlocked almost at the end of the game, it just makes Tabby move faster and is the least necessary of them.

Go to the Artificial Desert and head to the room indicated in the image below (look at Tabby's position):



This room features a flying platform coming down from the top, with three enemies on board. The platform will come down as soon as you reach the dip at the centre of the room.



An easy way to deal with them is to position Tabby in the dip, wait for the platform to come down a bit and finally shoot upward (toward the base of the platform). The subsequent explosion usually destroys at least two enemies, with a good amount of coins dropped. Don't worry about the remaining enemy on the platform (it will fly away with the platform), and more importantly be careful not to destroy the flying platform by attacking again.

At this point, just walk/jump a little in the room to collect the dropped coins, then exit and re-enter the room (left or right, it doesn't make any difference): you'll find that the flying platform will come down again from the top of the room, with three enemies on it. Therefore, just keep "visiting" the room, and rinse and repeat to farm some easy coins: (1) destroy the enemies on the platform, (2) pick the dropped coins, (3) exit the room and (4) re-enter the room.

Here are some tips:

• If you accidentally destroy the flying platform, just move away from the current room and visit a few rooms nearby. Taking the right exit and going to the room with the save point should work.
  
• The Artificial Desert features many rooms suitable for farming coins. I chose that room because it's easy to reach, the enemies are not dangerous, and there's a save point nearby. Feel free to explore the area by yourself, any suggestion is welcome.
  
• Being consistent is the key. Some visits will obviously be worse than others, because of inaccurate shots or missed coins (or accumulated boredom). It's better to take a break in case of bad performance.

For reference, I was able to settle on an average of 10 coins per visit, and about 100 coins per minute.

Thanks for reading this far, I really appreciate it!
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