You must complete 5 playthroughs, beginning to end, with each starting location for this achievement.
On Easy difficulty, I usually complete the game in 20-25 missions. Although you can finish it in 15 missions if you are dedicated. For Classic and Impossible, it usually takes me 25-30 missions before I can finish the game. I recommend you reserve the 2 best starting locations for beating the game on Ironman Classic and the Impossible difficulty achievements.
Each starting location has different bonuses.
1) Africa - Monthly XCOM funding increased by 30%.
2) Asia - All projects in The Foundry and the Officer Training School cost 50% less.
3) Europe - Labs and Workshops cost 50% less to build and maintain.
4) North America - All aircraft and aircraft weapons cost 50% less to purchase, build and maintain.
5) S. America - Autopsies and Interrogations are completed instantly.
NOTE: If you can't remember which starting continents you chose, you can see your base on the map in the situation room. It's marked with the XCom logo. Or, go to mission control and look at the globe. Your base is front and center. -Lt. Davos
I suggest the following order
1) Europe - Start with the Tutorial on easy and choose Europe. Knock out all the easy achievements
2) Asia - on easy, no tutorial
3) S. America - easy, no tutorial
4) Africa - Impossible (can't use tutorial on this difficulty). Then knock out
The Wet Work achievement in XCOM: Enemy Unknown worth 56 pointsComplete a Very Hard abduction mission in five turns or less on Classic or Impossible difficulty. The Lone Wolf achievement in XCOM: Enemy Unknown worth 118 pointsClear a UFO crash site with one soldier on Classic or Impossible difficulty.
I highly recommend you follow my guide (link below) for the Impossible difficulty. It will save you a lot of headaches.
By now, you should have knocked out every achievement except
The last achievement requires you to do Ironman mode which means you can't abuse the save/load system. You can however dashboard (or signout) but it's very time consuming. I also recommend you follow my Ironman guide.
5) N. America - Classic Ironman. Use tutorial for extra cash. And follow my Impossible guide to avoid headaches.