There are four categories of boosts (Melee, Power, Defense and Special) and they can only be found in the Bonus missions. They come in Bronze, Silver and Gold with gold being the most powerful and to get them you must score the indicated amount of points in the corresponding Bonus mission.
Distinguished Leadership Medal (Bronze): Score 26,000 points in the Defend Wakanda Bonus mission.
Commandeering Expertise Ribbon (Bronze): Score 40,000 points in the recruit Green Goblin mission.
World War Hulk Service Medal (Bronze): Score 48,000 points in the Wakanda Pillar Defense Bonus mission.
Meritorious Training Award (Bronze): Score 50 points in the Hack Challenge Bonus mission.
Medal of Marksmanship (Silver): Score 35,000 points in the recruit Gambit mission.
Outstanding Blitz Award (Silver): Score 25,000 points in the recruit Thor mission.
Outstanding Efficiency Award (Gold): Score 17,000 points in the recruit Songbird mission.
If you want to unlock ALL the characters without playing through the game completing tasks then press Up(2), Down(2), Left(3), Right at the 'Start' or 'Hero Management' menu. Enter the code again to disable the cheat.