StarCraft II: Wings of Liberty/Zero Hour

As briefed during the introduction cutscene, your base, now with the retrieved artifact, is under siege by a huge Zerg force. You must hold out for 20 minutes until rescue arrives.

If you're going for the Hurry Up: It's Raid Night achievement, you won't have to play this level again. The final play time for this mission varies slightly by a few seconds, but is just over 20 minutes.

Defend until evacuation
Periodically throughout the mission a wave of Zerg units will be unleashed. These are not random; each is given an exact number of units and are released at exactly the same time each play through. On Normal difficulty or below, it is not necessary to keep track of when each wave comes. On higher difficulties, particularly if seeking the third and final achievement, knowing when each wave comes offers an opening for offensive action and a way to better manage your units.

Waves switch from side to side and will rarely be unleashed on both sides at the same time (this occurs around 4 minutes remaining). Each wave primarily is made up of Zerglings, but as the level progresses, each wave will be harder by including more units and stronger ones (usually 1-2 hydralisks, sometimes 1-2 roaches, and later both)

There are also some smaller waves that offer alternative challenges, for example groups of three mutalisks are released occasionally (about 3 of these groups will be sent to bother you).
 * Mutalisk waves
 * The first mutalisk group will go after the top center missile turret. Bring an SCV and some marines to keep it alive.
 * The second mustalisk group will go after the bottom left bunker. You should have a Missile Turret on the bottom left side, below where a second bunker could be placed.

Periodically Zerg vessels fall from the sky. Some of these are just for show and end up exploding, however many of them produce a few zerglings or a creep tumor. If left alone, these areas of creep will be used to build spine crawlers, which will impede your attempts at rescuing the groups of stranded rebels.
 * Creep tumor "waves"

Rescue the rebels
At specific times during the evacuation, a small group of Mar Sara Rebels will appear that can be rescued. These are vital if you are going for the final achievement for the level. If these groups are not rescued within a few minutes, the nearby Zerg will attack and kill them.

Build order

 * 1) Refinery
 * 2) 5 SCVs and 1 Marine
 * 3) 5 SCVs
 * 4) Storage Depot
 * 5) Barracks 1 Tech Lab
 * 6) Medic and Marines (first wave arrives around here)
 * 7) Barracks 2 Reactor
 * 8) Top bridge bunker (first mutalisk wave arrives about here)
 * 9) Bottom bridge bunker
 * 10) 3 SCVs (you should have one for each bunker)
 * 11) Upgrade Infantry Weapons and Armor
 * 12) Bottom bridge Missile Turret
 * 13) Continual marine and medic reinforcements and storage depots

Defense tips

 * Build a refinery using one of the starting SCVs.
 * Build two additional bunkers near the bridges.
 * Do not build a third Barracks; there are not enough mineral deposits to make it efficient.
 * Fill the bunkers by the bridges. The two rear bunkers do not need to be filled unless you are doing poorly.
 * Continually train Marines and Medics for the duration of the map.
 * Before researching the upgrades, grab mineral and vespene deposits to pay for them. Each pair of deposits grants enough resources for one of the upgrades.
 * Be slightly offensive (more so if you are going for the third/final achievement); rescue the rebels and clear the road of creep tumors before they build spine crawlers.
 * Wait until the last three or four minutes to build more than one additional missile turret (remember to place one early on at the bottom left side of the left bridge, beyond where you would place a bunker). Mutalisks should not be a significant threat.
 * Learn to micro manage your grouped units so that none die. Hold alt during a skirmish to keep track of which units are taking damage. Marines that are being targeted by hydralisks, roaches, mutalisks, spine crawlers, or any other ranged unit can be saved by moving them slightly (even if it is still in range of the enemy, although this is dangerous and the unit may be re-targeted and killed). If your units are spread out, move the targeted/damaged unit backwards a step. If your units are bunched together, move the damaged unit sideways or forward a step (less likely to work).
 * Reduce the number of damage your buildings take, to reduce the resources wasted on repairs, by putting extra marines in the line of fire (with medics close by).

Achievement strategy: The Best Defense...
As the name suggests (the saying, "the best defense is a good offense"), you will have to go all out on this one. This is supposed to be a survival mission, but here Blizzard wants you to take out large portions of the enemy base. In addition, the difficulty must be set to Hard, which makes skirmishes much more tough and the AI more efficient and offensive. Don't be discouraged if this achievement takes you a few hours and a couple of full restarts. Saving your game periodically with good labels, including which attempt it is, is recommended.

Go into this thinking that you will not survive, and will not push the enemy bases until 10 or 11 minutes are left on the clock. You will get the achievement immediately after destroying the fourth hatchery. It is possible to get the achievement and complete the level, without losing the area behind the bridges. Also, do not attack the Zerg bases prematurely. You should take one large force that will be able to destroy the first base alone. Grab reinforcements when you move on to the top left base. Before the final push, your main force should be on defense for waves, and between waves they should be walking along the road to clear creep tumors and spine crawlers.

There are 8 hatcheries in the level, so you have to destroy half of them. They are split up amongst the three Zerg bases. The bottom left and top right each have 3, and the top left has two. Therefore, you have to take out one of the three-hive bases and a portion of the two-hive base.

To do this, you will have to be very fast and extremely efficient. Having just enough minerals is vital. At the start of the mission, make sure you build the refinery with one of the starting SCVs.

With the initial marines given, you can clear out the road above the top bridge that leads to the first rebel group. Leave four behind to man the bottom bunker, and take 12 up (hotkey 1). Make sure you are training marines to help aid the bottom bunker (on Hard, four guys can barely hold off the initial wave; the bunker may die). You should have 1-2 extra marines guarding the bottom bridge when the first wave arrives. After the initial push outside of the base, do not leave the base without medics.

Continue with the build order or one you have come up with, but make sure to spend efficiently. Do not lose units or buildings and reduce the amount of repairs you need to do (let marines take some damage and then have them retreat to be healed).

When rebel groups appear, go to save them after and between waves. Make sure to collect the resource deposits, then immediately research an upgrade. You must have both upgrades before attacking the Zerg bases.

The key to this mission is managing both time and money. To make it easy to move outside of the base, it is essential to clear out any Zerg forces, particularly those around creep tumors, before spine crawlers are built.

Until you launch your final attempt (make sure to save!), the roads just beyond the bridges should be clear of excess enemy units.

When you feel you are ready, around 10 or 11 minutes left on the clock and a wave has just passed, gather your main group and head to the bottom left base (the top right has an additional spine crawler and more units). Your group should have at least 30 units (about 1 medic per 4 marines). You should have a second group forming (hotkey 2) to help defend the base and reinforce your main group between base assaults.
 * Final push

During your pushes, retreat whenever you attract a large group of units. Do not engage spine crawlers if there are more than a few units coming after you, and never if there are a few strong units (mutalisks and hydralisks). You must micro your units (mostly marines) as best you can to minimize deaths. You cannot move all of your units at once; hold alt and quickly move your units slightly as they take damage.

When you have destroyed the three hives, retreat. Do not spend extra time on the buildings here; there are additional spine crawlers that do not need to be tackled. Gather additional forces and head to the top base. Repeat the strategy used before to destroy the base. Do not worry if the enemy has fully upgraded Zerglings (wings) by the time you reach the top base. If your forces are reinforced enough (~20 units), you should be able to handle them. Be patient. You only need to destroy one or two spine crawlers before you can attack the fourth hive. You should get the achievement between 3 and 5 minutes left on the clock (fully upgraded Zerglings appear around ~5:30.

Congratulations if you get it.

Secret: LVL80TSM!
Behind the top left Zerg base, above the ramp that connects to the top right base, there is a small area that has no creep on it and three trees. Behind it is a large pit. On this small area there is a neutral character called a Tauren Space Marine (a Warcraft/StarCraft hybrid reference). When you approach him, he will turn around and enter a portable toilet. Click on the toilet three times; each time you will notice some text counting down. Upon the third click, the toilet will blast off and text will appear noting that a secret has been found.