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Protoss vs. Zerg is an interesting matchup. The two races will employ similar tactics, that they will expand as much as possible, they will try to mass up an army as fast as possible with the highest tech level they have, and they will try to harass each other to slow resource gathering. However, the units at their disposal will differ, and as a result, their tactics will differ as well. This page will discuss what Protoss can do to defeat Zerg, and vice versa.
An experienced player will know how important it is to get a healthy economy. With a healthy economy comes plenty of resources, and with plenty of resources comes plenty of possibilities for the player, such as building more Gateways or Robotics Facilities, building more fighting units, building structures required to have more powerful units, and so on. Therefore, the primary concern of the Protoss is how aggressively they can expand (build new Nexuses with Probes mining minerals and gas) and how well they can protect these expansions. Without a healthy economy, a player is toast. Always.
That being said, an optimal mining base will consist of a Nexus placed as close as possible to the resources, with 24 Probes mining minerals and 3 Probes at each Assimilator. If the expansion is in danger of being attacked, defenses may be set up. Many players will have their army near their bases so that they can effective guard them. Expanding aggressively is a viable option if the enemy is not likely to attack the expansion.
Another critical tactic a player must have is scouting. Scouting is the act of looking at your opponent's base or army to have an idea of their plan. This is important because if you know your opponent's strategy, then you can build units to counter the opponent. Your opponent is likely to scout you to have an idea of your plan, and he may build units to counter yours. Because of this, it is important to scout as much as possible.
Early game examples of Protoss scouting is sending a Probe when you reach 9 Psi into the possible base locations of your opponent. Once the Probe gets to the enemy, you can have a glance at what the opponent has in store. Is he building a new Hatchery at the nearest possible expansion location? If so, he is trying to get a healthy economy as quickly as possible to build an army faster or to get stronger units sooner. Now that you know what your opponent is planning, you can react with a variety of ways. You can build your own Nexus to keep up with your opponent. You can try to send fighting units into the opponent's base to hurt their economy by killing Drones or killing the new Hatchery. In the later game, the Protoss can have better ways of scouting than sending a Probe into the opponent's base. They can create a Hallucinated Phoenix to fly into the opponent's base to have a glance of their plan. They can build a Robotics Facility to send an Observer into their opponent's base. There are a variety of ways to scout, and because scouting is so important, a player must do it every so often to know their opponent's plan.
The above paragraph is an example of scouting's benefits. Here's another one: Player A sends a Probe when it reaches 9 Psi to Player B's base. He sees a Spawning Pool built/being built with only 6 Drones mining. Player A sees that because there's a Spawning Pool in the opponent's base and little to no economic advancement of the opponent, Player A realizes that he must be putting it all into his army. What will his army be? It would consist of Zerglings and, rarely, Queens. Now that Player A knows Player B's plan, he can set up a Gateway and have Zealots and Probes ready to deal with the Zerglings. Now Player A has an advantage because he was trying to advance his economy, while player B is at a disadvantage because he has no economic advancement, and instead invested it all into his army, which died. Player A can send his own army in to try to kill Player B. Player A can try to tech up or expand, knowing his opponent will be behind. Possibilities are endless, and because Player A scouted, he has an advantage over Player B.
Units are the mobile army members/economy workers of any base. They are a key figure in winning the game, as without units, there is no winning.
The Probe, as with all workers, is the bread and butter of the Protoss. If the Zerg decides to do a Zergling rush, Probes are a viable defense in the early game. Care should be taken when entering the middle game, as a common Zerg tactic is to send units such as Mutalisks or Banelings/Zerglings inside of Overlords to kill Probes. As such, one must be careful after scouting a potential harass.
The Zealot (StarCraft II) is the first combat unit the Protoss can acquire. It is a powerful unit, but can be defeated rather easily if inadequately handled. In the early game, they serve as a possible way to defend your units and structures from early Zerg forces such as Zerglings, and as a way to kill the opposing Zerg's Drones or Queens if the Protoss player can afford it. In the later game, Zealots are used to tank damage thanks to their high health for such a low cost. Their main weakness is their slow speed without Charge and their attacks being melee range.
The Stalker is a ranged combat unit. It is fast, but is somewhat fragile. However, they can take damage quite effectively, and can be used as a tank if the situation calls for it (Stalkers should not be used as a tank all the time because of their high cost). They pair well with Zealots, as the Zealots take and deal damage in the front while the Stalkers do damage from the back. In the later game, they are used as a way to attack air units, and are one of the primary anti-air units the Protoss has.
The Sentry is a ranged support unit. Although they have a ranged attack, it should not be its primary tool as it has low damage. Instead, its primary usage is its powerful spells that can potentially decide a battle. Its Force Field creates a prismatic barrier that forcibly positions both allies and enemies. This is its primary usage, as the force field can cut a wave of attackers such as Roaches or Zerglings in half, rendering them unable to reach their targets. It can block a ramp, letting the ranged units do the work (units on lower ground cannot see up onto higher ground). The possibilities are enormous. However, they can be crushed by units with the Massive attribute. However, since the only massive Zerg unit is the Ultralisk, and the Ultralisk is easily defeated by Immortals or Void Rays, the Protoss need not be concerned. The Guardian shield is its secondary spell, as it reduces all ranged damage by 2. This may not seem like much, but since ranged attacks vary from 7 damage to 20 damage approximately, it can reduce ranged effectiveness significantly. For instance, a Marine deals about 7 damage. A Zealot normally can take 22 hits from a Marine before the Zealot dies. However, with Guardian Shield, a Zealot can now take 30 hits, 8 hits more. Guardian Shield can allow Zealots to survive long enough to reach the front lines, or generally to protect your units. The Hallucination spell requires prior research at the Cybernetics Core, and when it is fully researched, allows you to create a fake unit of your choice–provided it's available. The fake unit will attack normally, but the attack deals no damage. The hallucination will take double damage, and will disappear after a short time. Hallucinated units to you, and opponents with Detector units, have a purple tint, while to opponents without Detectors they will appear normally. Hallucination can create units to take damage, to scout, and possibly make your opponent leave early with the sheer amount of (fake) units you have.
The Observer is a permanently cloaked (meaning enemy units cannot attack it without Detector assistance) unit. The Observer itself is also a detector. It can be used to detect enemy cloaked/burrowed units or as a scouting units.
This is work in progress.