Starting Position - Protoss Base � Your base is vulnerable to attacks from West and North. Your resources won't last for long, and you'll be forced to move on the plateau above, so don't build any advanced structures here.
Starting Position - Terran Base � After a short moment, you'll be given command of a Terran base. You'll be given access to all of their units in this mission. Additionally, they are given a separate unit cap, allowing you to field a really big army with each of your factions. The resources are shared. The Terrans have a large amount resources at their disposal, so you can build a nice large base here. The Zerg will use mostly land units, and attack from the South.
Red Zerg mining base � This small base has only 2-3 AA turrets and a handful of defenders. Your assault, however, must be swift and decisive, as the enemy will send in reinforcements quickly. He will also try to rebuild his foothold in this area, so if you're not planning on expanding here yourself, you should keep some units around here, e.g. Siege Tanks on the hills or fliers near the mineral field (the Zerg will send workers in an attempt to build a new Hatchery).
Large Red Zerg Base � The Zerg have large bases defended by numerous units of all types and a good amount of turrets. There a Hive in the center of this location. You can try to destroy the main building or eliminate the rest of tech structures (preventing the enemy from training more units of a specific type). There are many ways to destroy those bases, but it's a good idea to attack both of the at once. This way, the enemy will be unable to send reinforcements from one base to save the other.
Siege Tank position � Each of these positions provides a good field of view and places many enemy structures inside the firing range of Siege Tanks placed in artillery mode.
Attack point of the Protoss ground force � Gather a squad of Dragoons, Reavers, Archons. Assuming they are at full strength, they will deal with most enemies before they can come close. There may be some problems with the Defilers, but the Protoss have Shields that will protect them from death. Additionally, you can take a squad of Zealots, Scouts, and (if you've got a surplus of resources) Carriers.
attack point of the Terran army � The Terrans should prepare a squad of Goliaths and Siege Tanks. Additionally, you train a group of Battlecruisers, but the mainstay of your army should consist of Ghosts and nuclear missiles. Goliaths will cover your army and special forces when they're locking onto their targets. This will enable you to move forward, missile by missile. Build several silos to avoid having to wait for nuclear strikes.
Purple Zerg Hive � The inner Zerg circle has 4 Hives, which will constantly train new units. Before you attack the center of the base, take out these structures to stop them from spawning new enemies.
Mission Objective - Overmind � Destruction of the Overmind will complete the mission. You don't have to wreck the base surrounding it to win. Although, eliminating most of the enemy forces in the area will be helpful. The Overmind has a ton of HP, but it cannot attack.