On the Fairy Tail Wikia, on the page Magic Staves, it says that Mystogan defeated all of the Phantom Guild's subdivisions by himself. However, I don't remember when this occurred. Which episodes was this from?
From this information, I have come to the conclusion that the episode would most likely be during the Phantom Guild Arc, but I still can't find it.
Mystogan defeated the subdivisions of Phantom Lord during the Phantom Lord Arc.
Manga chapters 47-69 and Anime episodes 21-30.
This is a snapshot from chapter 65 of the manga : Fairy Law. The anime should too have it in the same episode named Fairy Law.
Quote from the wiki linked above:
Just then, she notices Mystogan sitting quietly not far from her, who asks her for apples, and she realizes that the reason Makarov managed to heal so quickly was because the mysterious S-Class Mage gathered his scattered Magic power. She proceeds to lecture him for not joining the battle and helping out his comrades, but then the wind starts blowing and scatters numerous flags with Phantom's emblem on them around. Porlyusica realizes Mystogan did more than enough by taking out all of Phantom's subdivisions single-handedly, and the two remain still, discussing the situation enigmatically.
Note: Emphasis mine.
