Quickly go for the couplings while taking 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. Right 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 so that he will hit it and damage it. After a couple times of doing this, not only will you defeat Horgan but also get the hero objective of destroying the cannon.