At last, the final mission is here. Time for an all-out assault on the citadel and personal guard of Arcturus Mengsk.
The mission opens with Kerrigan in a starting base at the Southwest corner of the map. You have a modest force for self-defense already in place, but Arcturus opens up with a pair of attacks at the sides of your base- first the Northwest side, then the Southeast. Protect your encampment, and run up your mineral and gas income ASAP, because you're going to need every unit you can get your claws on.
A short way into the mission, a battleship lowers itself into the map, taking out a Dominion base and placeing several structures- Jim is here to support Sarah in her attack against Mengsk. This base is important, as it not only provides 'automated' attackers, but it contains Jim Raynor and the Hyperion. You need to ensure that the base remains intact at all times, because the destruction of the Hyperion will mean failure of the entire mission.
Jim's camp comes under attack immediately, but you'll need to let him handle it for now- Arcturus attacks again from the East. If you follow the attack back, you can quickly capture a nice secondary base location there, where you can double your mineral and gas output, vastly improving your assault rate. Beware of the enemy 'mechs there, these standing vehicles are much nastier than ground-mode valkyries (though not as tough as thors, thankfully) and will rip into light forces pretty hard.
An additional small outpost waits to the Northeast of the secondary base camp, which you will need to wipe out in order to cut down on the rate of Dominion attacks against your resourcing operation. Once you've done that, move out to Raynor's base- Arcturus has mobilized a flying assault force against it (or will soon) and Jim will need your help to fend it off, as his base isn't quite ready to supply his own attacks yet. With that handled, you may as well continue to the Northeast from there and take out another secondary Dominion encampment.