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.