Silvermane

Aunt May barely has time to discover this revelation before agents of the criminal Silvermane arrive and launched a rocket.

Right after they're defeated, Gepard pops in with a Gunner and a Cannoneer, these are the Silvermane Guards that need to stay alive for the achievement.

Granted, even with his deadly cyborg body, Silvermane isnt going to get a membership invite from the Sinister Six or anything.