Tiny Tina’s Wonderlands: How to complete the Alchemy Miracle Growth side quest

The Goblin can be found just outside the kelp you need to bypass in order to continue the story, blocking you from accessing Wargtooth Shallows. You'll get the Alchemy: Miracle Growth side quest if you speak with them. You'll need to try out the potion in some nearby kelp once you've received the mission. It won't be you who is obstructing your way. Turn around and engage with the kelp near the coral rocks instead. It will turn into a rock once you engage with it, and you will need to submit your results to the Alchemist again.

To perfect the potion, the Alchemist will report that he requires a special component. You'll need to look for it in a nearby dungeon, which you can discover behind you behind a small mound of kelp. On the trip to the dungeon, you shouldn't be assaulted by anything, but expect a lot of fighting once you arrive.

Inside the dungeon, proceed. You'll have to fight your way through two waves of adversaries. After defeating the final wave, a chest will appear, containing the final potion ingredient, and you will be able to exit the dungeon. Return to the Alchemist and give them the final ingredient, after which you can destroy the kelp blocking your path and move on to the next task.