This mission's time is based on 2 things right at the start. IMMEDIATELY TAKE THE FIRST TURRET ON PRISON BEFORE IT KNOCKS YOUR AIM OFF AND SPEED UP AND TAKE DOWN THE OTHER 2 ON THE OTHER ISLANDS. Your time will end based on how fast you take down the 3 turrets so the rescue boats can go in. After that destroy the gate and take down any threat to the rescue boats.

The mission ends in about 7 minutes. As of now, there's not really much else you can do.