This secret does or atleast currently not give you an achievement. It is just an easter egg which gives you an small bonus. I did also not find this secret.

Exactly what it is. It's a tomato which can be found right before the boss. The last shop you would encounter gets replaced with a room with a tomato inside and this song plays. This tomato refills your HP and increases it to 8 and gives you 10 extra charge.

Also, when you get to 3-1 (Aquifer 1) the first enemy you'll see is an happy little white jellyfish. This is also part of the easter egg and it doesn't normally show up in both modes.

To get to the hidden tomato you'll need to beat the four floors (Caverns, Catacombs, Aquifer and Limbo) without picking up any weapon or going to a shop. The bonus you get after beating a stage does not count.

• If you want to get this easter egg easy, then play it on normal mode.
  
  
• Get through the stages as quick as possible. Do not spend your time trying to earn gems (unless that's really what you want) since you are not going to need them when shopping. You can still get things such as the bonus which shoots a bullet if you collect a gem for example.
  
  
• Try not to get hit at all. Personally I wouldn't bother getting hit in the Caverns once but losing more than 1 HP in the Caverns means a reset to me. A very helpful item to stay alive with is the Knife and Fork. Since you can now eat certain enemies it also gives you extra health once in a while. Another item you should get early on is the Apple. Especially if you didn't get hit in Caves stage 1, get it early since it pretty much gives you 1 free HP.
  
  
• If you run out of charges, try to spam the charge button. It slows you down. There's still a high chance you might get hit by an enemy though, but if you stomp on their heads or hit the floor your charge refills.
  
  
• Choose either the flying style or the boulder style. Flying style gives you 4 HP but allows for easier movement and combo's, which can refill your HP after getting a combo of 35 and up. However the boulder style gives you 6 HP which is pretty sweet. Personally I prefer to use the boulder style.
  
  
• If you are good at Downwell, try to make combo's. Like I mentioned above, it can give you extra health if you have a combo of 35 and up. However, it is also very risky. You should skip this step if you have the Knife and Fork since that give you extra health much faster.
  
  
• Remember that the difficulty goes up pretty high when you get to Aquifer and especially Limbo. Take the time in Aquifer to eat up enemies while getting extra air and try to dodge as much enemies as you can while killing them too.
  
  
• Atleast have finished the game once. This is not something you might get on your first try. If you have completed Downwell atleast once then it shouldn't take too long before you get the tomato easter egg. You might still have troubles in the later stages but with enough skill (and luck) you will eventually break through.

And that's all I have to say. I hope this guide has helped you and has given you new tips as well. May you get the holy tomato!