How to survive the base defense mission?

Solution 1:

Precautions

This mission starts without a warning and you have to fight with the first six of your soldiers with whatever gear they have equipped right now. For that reason you should always make sure that these six soldiers always have at least decent equipment, even when you don't want to take them on missions right now. Otherwise you might have to face some strong aliens without armor and with conventional guns, which is not fun.

Mission mechanics

You will start with your first three soldiers and three guards in the command center.

Every odd round, a new wave of aliens will spawn. The aliens which enter the base with each wave are randomized. When you are lucky, you face lots of Sectoids and Floaters. When you have bad luck, you get killer-waves consisting mostly of Cyberdisks and Mechoids shielded by Sectoid Commanders. That means how difficult this mission is depends on how much the random number god hates you today.

The new waves will spawn:

1. in the command center
2. in the guard station
3. two times in the mech bay
4. two times in the access tunnels

Every even round, a new one of your soldiers will enter the map. When you already lost some guards, they will be accompanied by up to two new ones. This will happen three times.

Base Layout

The map layout of this mission has nothing to do with how you built your base.

   +-------+
   | Guard |
   |Station|
 +-+---  --+-+
 |           |
 |  Command  |
 |  Center   |
 |  (Start)  |
 |           |
 +-----  ----+
 |           |
 |           <- Reinforcement Entry Point
 |           |
 +----   ----+
 |           |
 |    Mech   |
 |    Bay    |
 |           |
 |           |
 +----   ----+
 |           |
 |  Access   |
 |  Tunnels  |
 |           |
 |           |
 +-  -----  -+

Strategy

The guards have normal assault rifles, terrible accuracy, only 5 hit points and tend to panic quickly, so they aren't much of a help. Their strongest weapon are the two grenades each of them carries. Use them soon - they do about as much damage as their assault rifles, but never miss and destroy the enemies cover. There is no penalty for causing colateral damage to the base interior, so there is no reason to worry about breaking stuff. When they used up their grenades, their best use is as cannon fodder to distract the aliens from your real soldiers. When one of your soldiers ends up in a bad position, move a guard into an even worse position so the aliens kill him first.

The beginning of the mission is the most difficult part, because you will have to face some strong aliens which enter close without warning and your force is limited.

The first thing you should do is get on higher ground. This increases your hit chance and reduces that of the enemies. It also allows you to hide from the Mechoids, because they can't climb ladders. The best place to concentrate your forces are the catwalks on the upper level between command center and mech bay. This location is easy to defend, close to the entry-point of your reinforcements and doesn't require you to shift your position much when the entry-points of the aliens shift further to the back of the base.

After you defeated the first wave which comes down in the mech bay, the mission will get considerably easier, because from then on you will have time to prepare properly for the last waves to arrive.

Solution 2:

If you're having a really hard time with the mission, try this:

go to the last saved game you have before the invasion. It might be the final round of a previous mission. As soon as you can (like after re-completing that mission), equip all of your other soldiers with stray weapon upgrades. I had several exalt lasers sitting in a pile, and simply equipping those made the mission much easier than trying to do it with the base weaponry. You can also make more armors and such. The invasion should happen the same day and time it did before.

Also worth experimenting is taking note of the soldiers in the room at the time. If they are the same after a reload from an earlier save, you would know exactly who needs equipment. I've noticed that even with save scum checked, abduction sites are locked in before they happen, so that may be true with the base invasion.

My experience with the waves is that they are the same at least in the same game - I used my above strategy to actually be able to beat it from a terrible defeat (I used the lovely option to free items before the previous mission, which makes this mission hell), and noticed that the waves spawned exactly the same troops in the same locations, even though save scum was enabled. This can give you a heads up so you can know where to position your soldiers (I went from the balcony of the hologlobe room in the first half to the balcony of the mech bay in the second half, where they had to walk through my overwatch.