Pixelmon: Shroom Trainers
Large mushroom structures can appear in the Overworld. Their resident Shroom Trainer guards a repeatable challenge and a unique progression item.
The challenge
Enter prepared and defeat the inhabitant to earn a Shroom. Unlike daily Gym Leaders, Shroom Trainers can be challenged again until you defeat them, so a failed attempt is information rather than a lost opportunity.
Turning Shrooms into progress
Exchange Shrooms on the Battle Point quest page. The reward provides Battle Points that can be spent on Pixelmon progression and choice rewards.
Preserve the arena
Mark the structure and avoid damaging the trainer area. A reliable repeatable encounter is more valuable than the building blocks.