• playing enemy unknown first and than within is great experience but in this guide we don't have a time for this so start with Enemy Within right from the begining.
  
• the tutorial is great for faster leveling your troops but it is only available for Europe and North America
  
• Slingshot DLC is great adition and you should have it enabled

• starting location - EUROPE
  
• tutorial - ENABLED
  
• difficulty - EASY/NORMAL

During your first gameplay you shoudl learn the basics and how the game works. I recomend to go with Normal difficulty becuase from Easy to Classic is very big step and one you'll need to make.
Also this should be your longest gameplay. Try to finish all hard achievements that require no difficulty such as:

• Ain't no Cavalry Comin' - Have a soldier survive every mission in a full game
  Just use one soldier in every mission (sniper is best choice) and do not let him die. If he is injured, don´t start mission untill he is recovered.
  
• Guardian of the Earth - Designate a highly decorated soldier as the Volunteer
  Just save 1 from every medal type and give it to "Volunteer" before last mission
  
• Xavier - Mind Control an Ethereal
  You need to have soldier with very high Will (use equipment to help raise will)
  
• Edison - In a single game complete every research project
  You need to interrogate every alien type as well (no robots ofc)
  
• All Aboard - complete Confouding Light with more than 3 turns left on the timer.
  You need to rush a little bit

Europe I think is the worst continent because you have 50% off from maintaining and building laboratories and workshops. I did multiple games without single laboratory or workshop so it is really useless. But it is your first gamplay and you plaing easy/normal and you can go with tutorial so Europe is good 1st choice.

• starting location - ASIA
  
• tutorial - DISABLED
  
• difficulty - CLASSIC

For your second gameplay you want to get used on Classic difficulty as much as possible and complete achievements that require this difficulty such as:

• Lone Wolf - Clear a UFO crash site with one soldier on Classic or Imposible difficulty
  Ofc you want to make this on Classic instead of Impossible. Bring your heavy soldier to first UFO crash site with only 4 sectoids and 1 outsider. Try to kill with granade 2 sectoids at time and save rocket launcher for outsider.
  
• Wet Work - Complete Very Hard abduction mission in five turns or less on Classic or Impossible difficulty
  Just start a mission. Save. Rush the whole map. Kill everyone. If some of your soldiers died reload save after you get achv.

You will not need tutorial anymore so you can go with ASIA with half prize of officer training school upgrades and foundry projects bonus, which is nice addition.

• starting location - AFRICA
  
• tutorial - DISABLED
  
• difficulty - IMPOSSIBLE

This will be your most challenging and hardest game. You will go just for achievement for completing game on Impossible difficulty and nothing else since this is the hardest achievement. The best place to start is Africa in my opinion, because you have 30% more income from every satelite and on impossible you are instantly broke. Unfortunately you can not start the tutorial in Africa (so you miss easy XP for your soldiers) but it is better to save North America with tutorial for later.

• starting location - SOUTH AMERICA
  
• tutorial - DISABLED
  
• difficulty - CLASSIC

Your fourth run will be on classic difficulty and you will aim it on An Army of Four achievement. Now this achievement has the second lowest % of players unlocking it (less then Our Finest Hour - Beat the game on Impossible difficulty) but it is not that hard. Everything you have to (not) do is to not buy the squad size I and squad size II upgrade leaving you with only 4 soldiers per mission instead of 6 the whole game. Ofc this needs to be done on Classic difficulty (at least). At this point you should be familiar with everything in the game and you already beat Impossible difficulty. Classic with 4 soldiers is like 10 times easier trust me. You will start in South America with "Aliens intorrogations and autospies are complete instantly" bonus. This is like second worst place to start after Europe mostly because it is so easy to cover South America with satelites and have this bonus at early point in game because there are only 2 countries there. But, this bonus is very useful for your 4 soldiers because you need to level a circle them as fast as possible and with this instant research you will have better items sooner making your soldiers better.

• starting location - NORTH AMERICA
  
• tutorial - ENABLED
  
• difficulty - CLASSIC

Your fifth and final run will be an Ironman mode on Classic difficulty. Ironman is game mode which uses 1 save file which overwrites after everything you make, so your choices are permanent as well as your soldiers deaths, your shots missed etc. You cannot "turn back" and reload last autosave. BUT there is an option to copy the save file where your Ironman is manually, so you can reload your save. This makes Ironman less terrifying than Impossible diff. But still it is boring and kind of cheating method. I have to admit that I used this method during my Ironman 2 times because I would lose my whole squad of best soldiers and probably the whole game, but it is totaly doable without it.
You will start in North America with 50% off to satelites and aircrafts bonus, which makes it perfect for satelite rush. You spend everything on satelites and workshops, pick abduction missions which grands you engeneers and if you survive first few months you are done here. Than just be careful. Do not rush and calculate your every move and this should continue as your basic 2nd Classic walkthrough. Also you can (and should) enable tutorial for this run so you will be granted free Heavy soldier right before your first mission which makes this even easier.

You got it. By now you should have every achievement completed in XCOM Enemy Unknown (achievements I did not mention should be completed as you progress through game, but watch out for any missing ones) maybe the 2 from multiplayer still left but that is 5 minutes matter.

So, congratulations for your next 100% game. If this guide helped you you can leave a like. If you would change something leave a comment. Also english is not my native language so grammar incorrections will be included :) Good luck, central out.