XCOM: Enemy Unknown

Take note of your starting location or keep a saved game file at the global map each time you are almost finished with a game in order to keep track of which locations you have already completed. Play under the easy difficulty setting and use second wave options. Because this must be done within the same game, either Enemy Unknown, or Enemy Within, select the one you have more completions with. If you choose Enemy Unknown, you can flank and overwatch an enemy and they will remain stationary (unless one of your soldiers is also flanked, in which case they will get shot). This cannot be done in Enemy Within. Use the Slingshot pack to get an advantage early on by capturing a live Muton and studying plasma weapons to get research credit for them. Additionally when you complete the final mission you can make the Blaster Launcher if you have enough fragments. The best method to get superhuman soldiers is to use the second wave option "Not Made Equal". Have the "Instantly promote to squaddie" and "Iron will" upgrades and hire at least seventy soldiers. Examine them and only upgrade those whose Aim is at least 80 and Will is at least 50 (60 recommended). Use them to build a squad that hits often and can mind control an enemy frequently. It is also possible to use the psycho test to get all "gifted" troops. Place three men into the test chambers, save the game, allow the test to complete, note the names of the ones that are gifted, reload the saved game, remove the non-gifted men, and repeat until you find three gifted soldiers. You can them spend some time upgrading them to max Psi level.

Take one of the survivors from the first mission into every battle until the game gas been completed. Consider using the sniper, as it usually operates from a distance and is safer than the other classes. Also do this in easy mode on the basic game (Enemy Unknown). Make sure your select soldier almost never gets hurt. If a new mission starts and your soldier is still injured, skip the mission.

Each of the following alien autopsies must be completed during the same playthrough. Note: Have a starting base at South America to get autopsies that are completed instantly.

Sectoid

Sectoid Comander

Sectopod

Thin Man

Muton

Muton Berserker

Muton Elite

Floater

Heavy Floater

Drone

Cyberdisk

Chryssalid

Ethereal

Do not attempt this in the default game. It must be done in Enemy Within.

Have one of your soldiers wearing Archangel Armor. The soldier must kill an enemy while flying. Archangel armor is an expensive late game armor that must be researched first. It helps the wearer to fly. Wearing Archangel Armor on a Sniper class soldier with Squadsight ability is the easiest way to earn ""Angel of Death".