StarCraft: Brood War/Protoss Mission 8: Countdown

Walkthrough
In this stage, you need to defend the temple at the middle of the map near 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. Zerg is cheap and strong, especially for the Hydralisk, and it is a great advantage that it can attack ground and air units with average damage. Zerg Nydus Canals are a good transfer for reinforcement units.

Photon Cannons are your best bet, supplemented later on by Corsairs and Scouts for the occasional Guardian attack as well as Queens who will try to spawn Broodlings on your non-robotic ground units.
 * Tighten up your base defense.

The most important part of this stage is to capture a Drone and build up a Zerg force in either 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. Also consider capturing Overlords as they are very good and cheap alternatives to both the Shuttle and the Observer. All enemy Zerg should have Overlords with transport, speed and sight upgrades, so mind-controlling an Overlord will save you a significant amount of time and money.
 * Capture a Drone.

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. Other than the ones at the temple, this is the only resource node that the enemy is not mining. There are a few static defense structures, however.
 * Capture the only natural extension.

Make a lot of drones to gather the extra resources and build a lot of Spore Colonies and Sunken Colonies.
 * Build two hatcheries beside the temple.

Spawn Hydralisks 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 turn into ashes.
 * Fill up the space around the temple.

Depending on how confident you are feeling, it IS possible to completely eliminate the Zerg from the map, but it won't be easy and it will take almost all of the available resources on the map. However, it will be worth it to basically win the level once your special units reach the temple.
 * Alternate advanced strategy

Build 3-4 Gateways, and mass Dragoons. Build 1 Stargate to build some Corsairs for base defense against Guardians. Build at least 2 Rob. Facilities for Shuttles. Don't bother with Scouts/Carriers, as scourges and hydras will eat them alive.

You should be able to do some serious damage with 2 teams of dragoons. The teams will die often, which is why this strategy can be expensive, but it is effective and cheap to replace (compared to most Protoss units). Make sure to bring at least 1 Observer, since Lurkers will rack up damage fast. You will also want to bring some Corsairs to Disruption Web the Spore Colonies that completely surround the brown and orange Zerg bases, otherwise you will lose Shuttles during your attacks.

This is a tricky strategy to pull off, as it requires good resource management and good unit control (Knowing when to focus attack, and how to get as many of your bulky Dragoons firing as possible) but for those looking for a challenge, or just a more straightforward approach to the mission, this is it.

If you are not comfortable using Protoss units to eliminate the Zerg, you can also use Zerg units. After you have mind-controlled a Drone with a Dark Archon and set up a Zerg base, you can use your supply of resources to build Guardians and Devourers. They are very expensive units, but will come in very handy when fighting another Zerg, especially in this map. Devourers are the Zerg air units that can only attack air, like Corsairs and Valkyries. Take a fleet of 36 Guardians and 24 Devourers and about 4 Overlords to the Orange base, where your attacks will begin. Starting with the Guardians, take down the Spore Colonies that carefully surround each island. Have the Devourers slightly behind to counter any attacks from Scourge. Manage your units carefully to minimize losses. Even though there is a plenty of resources at the temple, losing a large force is still incredibly painful in this mission. If you made it to the island with the combined Brown/Orange bases, be more cautious as Defilers will plague your units, making them easy meat for the Scourge or other enemy air units. If you have been plagued, move your units out of th way and let them sit and heal. In the mean time, you can build fresh units to reinforce your attack. If you are running low on resources, you can always take over the resources on the islands you destroyed. Even if you are 45 minutes into the game, the geysers in the Zerg bases will still have a large amount of gas left over. Since Guardians are longer ranged than Dragoons and cannot be attacked by Lurkers, Zerglings and Sunken Colonies, they will make your resource management a little easier even though they are more expensive to build. Nevertheless, you may have to replace your forces a few times as they will often get killed by Scourge.
 * Alternate advanced strategy 2

If you are using this strategy, build extra Probes to do all the resource gathering. Save the Zerg supply for the air-attack force.

Alternate Strategy
If you aren't comfortable with the Zerg units or don't want to manage another tech tree, then a simple all-Protoss strategy is to ring the temple with Photon Cannons. Lots and lots of Photon Cannons. They're very cost-effective at 150 minerals and 0 gas. However, guardians can destroy them from outside their range, so keep a team of scouts ready to pound any that appear. Whichever strategy you choose, resources are plentiful at the temple, so spend freely on your defenses.