Elektra (Chapter 2-3)
Use hero: Black Cat to defeat
Elektra Drops: Grecian Sai – counter and bleed effect
Magneto (Chapter 4-4)
Use hero: Storm
Magneto Drops: Magnetic Field Generator – creates shields and when the shields are removed the ally gains an extra turn
Dr. Doom (Chapter 3-5)
Use hero: Spiderman
Dr. Doom drops: The Doombringer – Debuff an enemy and raise your agents stats
Loki (Chapter 5-3)
Use hero: Wolverine
Loki drops: Norn Stone – Increase all the agents’s stats
Doctor Octopus (Chapter 6-3)
Use hero: Black Widow
Doctor Octopus Drops: Electrostatic Arm – Attacks will also shock enemies; enemies with negative charge receive extra damage
Have in mind that in order to actually get the Epic Bosses, you will have to complete all the missions in the chapter.