Achieved a combo streak of 42
Difficulty (Medium/Hard)

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This can easily be done during the combat challenges. Combat challenges can be started at the arcade machine in Stan's Comic Book Store (marked with a blue book-symbol on your map). In fact, while playing through all 12 challenges there is a good chance you will unlock this naturally. The easiest way, however, is to repeatedly attack the last remaining enemy of a round with the Web-Strike (RB). The Web-Strike doesn't actually do any damage, so you can keep doing this indefinitely.