This cumalative trophy goes across all heroes.
I earnt this during Iron Man's second solo mission in the main campaign.
Basically kill 50 enemies using any heroes super powers.
The powers are mapped to:
Defensive ability (Hulks is the only one that does damage upto the recruitment of Iron Man)
Offensive Ability
+
Ultimate ability. Hulks is the best for this trophy, Embiggen ramps does a lot of damage also, but early on Iron Mans abilities tend to only take out a couple at a time.
Hulks
Attack may not count, but thought I'd include it as I used it alot jist before the Iron Man missions. If it doesn't work for you, please comment so I can add an edit.