Load the “Raid On Oscorp” mission. Proceed through the mission until you leave the air vents, and a cutscene will begin, which can be skipped. Stealth kills four enemies and then disposed of the final enemy as desired.
After defeating the last enemy, pause the game. Select the “Restart” option, then choose “Yes” to restart from the last checkpoint, which is where you leave the air vents. Repeat this process until you reach the desired suit level.
Note: The max suit level is 8.