18 Janissaries arrow-stormed at once + Armor of Ishak Pasha (Assassin's Creed: Revelations)
All these Janissaries were still on the scene after completing the mission Sticky Situations, part of the Piri Reis bomb tutorial missions.
You won't be able to kill all the Janissaries this way if you replay the memory, since you will be brought back to the Animus DNA interface immediately after completing your objective.
The armor of Ishak Pasha is obtained by collecting all 10 Ishak Pasha pages and then exploring the Hagia Sophia (http://assassinscreed.wikia.com/wiki/Armor_of_Ishak_Pasha).
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