Below is a walkthrough for the Alchemy: To Block the Sun side quest in Tiny Tina’s Wonderlands. To obtain this quest, speak with Orson in the Overworld.
Reach valley
Travel to the objective to the north to find a cyclops.
Kill cyclops
Bump into him to trigger a battle. Complete the encounter.
Pick up cactus
Retrieve the cactus where you found the cyclops.
Talk to Orson
Return to him and speak with him to complete the quest.