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.