In this stage, you need to defend the temple at the middle of the map during the end of the game after two VIPs reach the temple. What you really need is a big bunch of armies surrounding the temple to make sure no one can reach it. To get those armies, you need to do the following step by step. You need Zerg forces, because otherwise you will end up bankrupt if you defend the temple with all Protoss units. Zerg is cheap and strong, specially for Hydralisk, and it is a great advantage that it can attack ground and air units with average damage. Zerg's cydus is a good transfer for reinforce the units.
Photon cannons, dragoons and some zealots. Just make sure you won't end up swallowed by the Zerg's minions.
The most important part of this stage is to capture a drone and build up a Zerg's force either in expanding base and defend the temple at the end of the game. Take over the red color Zerg base as soon as possible after you stabilize your base defense. Using mind control, capture a drone and make a base with it. Upgrade Hydralisk's stats, speed and range, to max.
On the east side of the map there are natural extension sites for you to gather the money you need for the defense of the temple.
You will waste your time and resources; the mission objective is to protect the temple for 15 minutes, not destroy all enemies' bases. Also, you will end up bankrupt by assaulting either the brown or orange zerg.
Make a lot of drones to gather the extra resources and build a lot spore colony and sunken colony.
Spawn a Hydralisk with all your resources and fill in the blank in the middle of the map. If you have extra resources, buy some Carriers and park them on the temple. Anything that comes near the temple will become ashes.