INFINITE TURN GLITCH
Works in base game only, it does not in War of The Choosen.
Achievement is quite easy with infinite turn glitch. This glitch works in standard and Ironman playthrough, on any difficulty.
To make it work you have to save game short before you finish last move of your last character after which normally enemy would have his turn and then load that save. You will still have your turn while enemy looses his.
Just be carefull with few exceptions, enemies who transforms from other forms or pop up can still make their move so watch out when you want to trigger them - this applies for Andromedons, Chryssalids and Faceless.
Using this glitch you can even go for all below achievements in same playthrough:
The The Few and the Proud achievement in XCOM 2 worth 335 pointsBeat the game on Commander+ difficulty without buying a Squad Size upgrade The Valhalla achievement in XCOM 2 worth 456 pointsBeat the game on Commander+ difficulty in Ironman mode The The Untouchables achievement in XCOM 2 worth 260 pointsBeat the game on Commander+ without losing a soldier