XCOM: The Base Assault

After a climactic and harried battle with the aliens, my X-Com team unearthed a massive alien base in Siberia. I hesitated, wondering if it was the end of the game or not. After building up my air force and ensuring I was properly equipped with plasma weaponry and decent armor, I made the jump.

Update: I just completed the mission on a higher difficulty (Impossible).  The layout was entirely different.  You still fight your way through a few rooms of enemies, and they are random, and it ends with the same Sectoid Commander.  All of my advice about squad still applies, as the map isn't fundamentally different, but the room advice is mostly bunk.  The enemies flex to what you see when you shoot UFOs down, though it might surprise you with a few robot mobs or Mutons if you go in immediately -- I hadn't encountered drones or Mutons in my Impossible play through, but both were in the base.
The first thing I should note: this is not, in fact, the end of the game. This is the midpoint, for sure -- it's a transition in many ways. The first is that you appear to be moving from playing defense to offense. You have what appears to be end-game technology, minus psionics. You should have access to Plasma, Archangel armor, and you should be familiar with basic effects. Still, the mission is particularly brutal -- I've heard players weeping over wiping on the mission even on normal mode (and for the record, I just completed Classic Ironman, and didn't lose a man).

 Squad Composition
There are a number of things you should note while playing this mission. Firstly, there are long, unobstructed lines of sight. If the words "Sniper's Playground" entered your mind, you're absolutely correct. Secondly, expect mechanical units -- bring a Heavy or two. I ran it with 2x Sniper 2x Heavy 1x Assault 1x Support. Were I to do it again, I'd drop the Support for a second Assault, as I was overly cautious.

Room 1
The initial room is empty, along with its wings. Ignore it. There is a door on each side of the room. Make a 3-man (or 2-man) door stack, and end your turn on overwatch. Enemies spawn behind these doors, so you want to use your first action of the turn to open the door.

Room 2
 Don't move in -- clear this first spawn from a range. There's a second spawn behind them -- Chrysalids on the left side, for me. Move in and engage, dropping the second spawn in a single turn. Star of the room: Heavies.

 Room 3
This is a room with three UFO power sources. There are no enemy spawns that are visible when you open the door. One move in, you'll notice two wings with what look like operating tables, and a center with what appear to be jars eminating a purple glow. There is a spawn in the right wing just as you reveal the third power source, and a spawn in the back of each wing (for me, one Sectopod and two Muton Elites).

Finally, there's a spawn (Light Muton, Muton Berserker) at the far end of the room. Note that crossing or standing in the purple glow does not harm you. Once you clear the room, stack your whole team on the exit door, and end the turn. Star of the Room: Snipers (with Heavies getting an honorable mention). The lines of sight are perfect for a sniper to just park on the balcony of the third power source. Heavies make quick work of the Sectopod.

Room 4
Just a holding room. Prepare for a massive room.
Room 5
A massive room with 3 levels, 2 wings, and a chokepoint leading to its end. Spawn on mid NE corner of the dip (Heavy Floaters), spawn just as you enter (Muton, Bers), and a final boss on the other side where there's a large red glow. Engage slow.  Don't go for the red beacon on the far side of the room until you've cleared the rest of the room. Be careful when you engage the boss -- he's weedy, but he's nasty if he gets the drop on you.  I used a sniper on the top level to reveal it, and used my second move to get away without engaging, kiting him back toward my squad.

The Boss Fight

This is your first taste of true Psionics -- a Sectoid Commander, with the ability to Mind Control. Once you reveal him, be prepared to pull back, kiting him backwards toward the entrance. I positioned snipers on the wings, and used my Assault/Support as bait. On Classic, he only had a handful of hit points -- enough that a single pistol shot brought him under three health, leaving my Support unit to capture him.  Enjoy finding out what his innards look like in your own little alien autopsy.