This guy's a pushover compared to the other supervillains. The first part of the fight is a simple beat-'em-up; roll dodge to avoid the grenades Black Mask lobs, and stay away from the center of the room so as not to get torn up by his gunfire. Next comes a gel detonation puzzle, where you have all the time in the world. After you catch up with Black Mask, you'll spot him standing under some lights; don't approach him, or you'll die instantly. Instead, chuck a batarang at the far right alarm, causing Black Mask to shoot in that direction like crazy.
Once he's distracted, immediately break the nearest light with a Batarang and hop into the floor grates. Walk underneath Black Mask all the way to the far right side, then hop out; he won't spot you in the darkness. Throw a Batarang at the alarm on the opposite side of the room, then duck into the floor grate and sneak towards the center grate. When Black Mask walks over you to investigate the noise, bust out a Takedown on his suited behind.
