This achievement is unlocked by beating the boss fight at the end of Veggie Village, the second world in the game. All levels need to be finished in order to unlock the boss fight but if you're going for the 100% you want to make sure that you have 22/22 clovers/pages, which can be checked at the top right of the screen. If you don't have 22/22 and aren't sure how to get the remaining pages you need, make sure to check out my guide for pages, which will be coming soon. If you don't want to do that yet, you can return to the world using the world select option in settings, so don't worry.
Once all 5 levels in Veggie Village are complete, you are ready to fight Tess. Once she's defeated, you will be able to progress to the third world, Wrestful Retreat. From the bottom of Veggie Village hub, you should be able to see a castle in the distance with Tess's face on it. Simply head towards this, and once you cross the final bridge and step into the portal you will get a cutscene and then a dialogue chain.
Once this is over, you will be fighting Tess. In order to defeat her, you will need to flip the switches that spawn in after she's tried to attack you, and this will deal damage to her. It's scripted, so there will be three "waves" and once all are done she'll be defeated.
To start off, Tess will charge at you. In order to avoid this simply head to the right. Once she has done that the first set of switches will spawn in. Simply flip all 4 using
from nearby, and this will attack her. You will also need to watch out for the lasers down the middle and the fireballs that start to come down each side, but you can easily get past them by double jumping. After Tess is damaged she will spawn 4 bees. Simply double jump onto there heads to kill them all. Once those are dead, wave 2 will begin. Tess will charge, and then hit a random side. The attack radius is very small and it is easy to dodge this, so just keep an eye out. Once that's over, the next wave of switches will spawn in. Simply repeat what you did last time, but there are more switches and lasers this time. Once Tess is attacked a second time, she will spawn 4 bees- once those are dead, her final attack will begin. She will charge, then hit both sides in a short space of time. As long as you dodge those, which is easy due to the small radius, just flip all the switches that spawn. After that, you have defeated Tess and the achievement is yours!