How to defeat The Hammer Falls
When I completed that campaign (Rebel Yell) in 2001, my strategy for the last mission “The Hammer 
Falls” was to build 12 Siege Tanks and 12 Goliaths to kill red Terran, then build 12 Battlecruisers to kill white Terran. Getting the Ion Cannon was very easy from there.
Try destroying the ion cannon.