Achievement and Gameplay Tips

Use the Steam Discussions post to find people who have the achievement. It'll unlock right when you start a game with someone with the achievement.

Don't attempt this too late into the game, because aliens get really tanky. I recommend getting it around the time you start seeing floaters, because they still get 1 shot by rockets.

I went 2 heavys, 2 snipers, 1 support and 1 assault. Save at the start of the mission and rush it as fast as possible. If you fail, reload and use rockets to destroy pods of aliens from fog of war.

Confounding Light ends right when the last objective is complete, so don't worry about wiping the aliens too much. Bring a support with Sprinter (3 extra movement), and run him to the end of the train while activating the transponders.

Some snipers with squad sight to cover or some assaults to clean up the sides of the train can be good too.

40 UFOs takes longer than you expect.

The "ally" is Shaojie Zhang who you get from the "Friends in Low Places" mission with the DLC Operation: Slingshot.

Unlocks after killing the Uber-Ethereal, assuming Shaojie Zhang is in the final mission. Not at the start of the final mission as others have said.

Lategame this is almost impossible. Do it as early as possible:

https://steamcommunity.com/sharedfiles/filedetails/?id=175141372

Cannot switch the difficulty at the very end or before the last mission, or during a penultimate mission, you need to play on Classic from the start.

Lategame, I recommend 2 snipers, 1 mimetic skin assault and 1 heavy. Play around the snipers and mimetic skin to win encounters.

For other gameplay tips, look at the Our Finest Hour section.

Cannot switch the difficulty at the very end or before the last mission, or during a penultimate mission, you need to play on Classic from the start.

For some gameplay tips, look at the "Our Finest Hour" section.

Definitely save scum for this, 9 damage crits just aren't fun to play against. You can save scum by copying the Ironman save located in "%USERPROFILE%\Documents\my games\XCOM - Enemy Within\XComGame\SaveData" to another location, then putting it back when you want to reload.

I've made a small repo with some scripts to help with this, feel free to use it:

https://github.com/Brissot/XComSaveScum

Also, look at the "Our Finest Hour" section for some gameplay tips.

Cannot switch the difficulty at the very end or before the last mission, or during a penultimate mission, you need to play on Impossible from the start. This will likely be your hardest playthrough, because you can save scum in Ironman.

Pregame
Second Wave makes the game playable:

• Save Scum: loading allows "rerolling" shots
  
• Absolutely Critical: flanking guarantees a crit
  
• Aiming Angles: aim bonus for flanking

Start in Africa for the 30% revenue boost.

Early Game
Goals (in order):

1. Satellite coverage
   
2. Path to thermo generators
   
3. Build an officer training school
   
4. Level up soldiers

Note about MECS: They're too expensive early game; it's better to spend the money on satellites, and save the meld for mimetic skin.

Research:

• Alien Capture: Xeno-Biology -> Arc Thrower -> build Alien Containment -> capture alien -> interrogate alien
  
• Carapace Armor: Capture floater (for the research credit[xcom.fandom.com]) -> Alien Materials -> Carapace Armor

Do not research Beam Weapons to save weapon fragments, and because laser weapons aren't a major upgrade from stock weapons, especially the class specific weapons: Shotgun, Minigun and Sniper Rifle.

Also, sell all bodies for the money, and don't waste time on autopsies until you get South America's bonus "We Have Ways".

Some countries will withdrawl from XCom. Choose 2 continents to lose: Africa (you can't lose the home bonus) and 1 other continent (I chose Asia).

Encounters: Use grenades to destroy cover. This is enough to count as flanking, so with Absolutely Critical and Aiming Angles you have a guaranteed hit/crit. Also, be liberal with rockets. Resources aren't as important as leveling up your soldiers.

Ideally, kill the whole pod of aliens the same turn they're revealed, so they don't get a chance to shoot. If you don't manage to, make sure all your soldiers are in full cover, because Aiming Angles and Absolutely Critical works both ways.

Midgame
Make your soldiers stronger with plasma equipment and stealth skin.

Research

• Plasma Weaponry: Weapon Fragments -> Capture Muton (for the research credit[xcom.fandom.com]) -> Light Plasma rifle -> Plasma Rifle -> Plasma Sniper -> Alloy Cannon -> Heavy Plasma Rifle
  
• Stealth: Meld Recombination -> Genetics Lab -> Mimetic Skin (Assaults, Supports)
  
• Objective: capture Outsider -> Outsider Shard -> buy Skeleton Key -> Alien Base assault -> XCom Base Defense

Save scum to find and destroy the Exalt HQ.

Encounters: Use mimetic skin to find pods of aliens in stealth, then use squad sight snipers to wipe them.

Lategame
Soldiers have the best gear, so story objectives.

Research:

• Firestorm: Alien Nav Computer -> UFO Power Source -> New Fighter Craft -> buy Firestorm -> EMP Cannon -> buy EMP Cannon
  
• "Chosen one": Sectoid Commander Autopsy -> build Psionic Lab -> test soldiers
  
• Story: Hyperwave Communication -> build Hyperwave Relay -> raid Overseer UFO -> Ethereal Device -> PSI Armor -> buy PSI Armor -> build Gollop Chamber -> Temple Ship

Soldier Upgrades:
Note: Lightning reflexes is better early
Note: Opportunist is better early
For the final room, you can position squad sight snipers outside the room to kill the Uber-Ethereal in one turn.

You can get Continental Fellow by just playing for all the other achievements. You can do any combination, but here's what I did:

1. "Blind" playthrough, easy, started in North America
   
2. "Buy/research everything" playthrough, easy, started in South America
   
3. "Army of Four" playthrough, classic, started in Europe
   
4. "Classic Ironman" playthrough, classic ironman, started in Asia
   
5. "Impossible" playthrough, impossible, started in Africa

Worst-case scenario though, use speedrun tactics to rush continents:

Originally posted by https://kb.speeddemosarchive.com/XCOM:_Enemy_Unknown_(2012):

Base Assault

• Research: Xeno-Biology -> (Arc Thrower, build Alien Containment) -> (capture alien -> interrogate alien, capture Outsider) -> Outsider Shard -> buy Skeleton Key
  
• Resources Required: 10 Weapon Fragments,10 Elerium, 10 Alloys

Overseer UFO

• Research: (Alien Nav Computer, UFO Power Source) -> New Fighter Craft -> (buy Firestorm, EMP Cannon -> buy EMP Cannon) -> Hyperwave Communication -> build Hyperwave Relay
  
• Resources Required: 4 UFO Flight Computers, 2 UFO Power Sources, 70 Elerium, 90 Alloys, 75 Weapon Fragments, 30 Engineers

Temple Ship

• Research: Ethereal Device -> (PSI Armor -> buy PSI Armor, build Gollop Chamber) -> Sectoid Commander Autopsy -> build Psionic Lab
  
• Resources Required: 60 Elerium, 40 Alloys