This is a very challenging mission, with anti-air defenses everywhere. Immediately go for the couplings. Take as little time as possible on defensive units. If you take your time destroying them and go on to other couplings, the couplings will be repaired and defenses will be reinforced. Immediately after their destruction, go to the center and destroy defenses. Hide under the platform to recharge your armor. Once the defenses are substantially diminished go to the top and strafe Horgan, keeping the cannon below and in front of you. When he charges his attack, drop behind the cannon. He will hit it and damage it. After a few times of doing this, not only will you defeat Horgan but also get the hero objective of destroying the cannon.