XCOM 2

OK so there's no solution already on here and I suppose there can't really be a 'solution' for this achievement as such, so I thought i'd post some helpful hints and advice.

The obvious stuff
-Ironman is a game mode where you can only save once. The game will save at certain points automatically (e.g. at the start of your turn during battle) and the autosave option makes no difference. If you fail the game for whatever reason (e.g. Avatar meter being full) you will fail the game entirely and will have to totally restart from the beginning potentially losing 20+ hours of gameplay.
-This can be done on commander or legend difficulty (if you're REALLY looking for a challenge)
-This can be done in combination with other achievements (e.g. only using 4 squad members, not letting a single solider die, etc) but this would only make it a lot harder and more frustrating so i'd strongly advise against it.
-You cannot change difficulty of the game at any point (it will warn you in-game changing difficulty will disable achievements)
-For the love of all that is good don't try this as your first playthrough (even if you've played xcom 1!)

Most important tips
-While you can't 'save and load' in the traditional sense, you can quit the game and reload back up. For example, if you're in a battle and your strongest character dies that you've named after your BFF/GF/BF/whatever and spent hours customising them to make them look super-cool. Quickly pause > hit the guide button > press start on xcom2 > quit. Reloading the game will bring you back to the start of your turn. You'll be doing this a lot.
-Take your time! Don't rush! Yes, some missions are timed but I still found i could take a turn to reload and position units strategically without running out of time.
-This also applies out of battle. Of course this isn't an achievement you want to go on for too long, the longer you take, the worse it'll be if you fail. However rushing the main missions may mean you're far into the story fighting higher tier enemies while you've only got 4 soldiers and tier 1 gear.
-Fight enemies, not armies! Never pull more than 3 or 4 enemies at time, you'll get overwhelmed (especially if game autosaves and you've got 5 half-dead soldiers versus 8 full health enemies). If you go too far forwards in a mission and end up pulling 3 groups of enemies it's probably worth quitting and reloading the game before the game autosaves.
-Hack, hack, hack! Hacking and disabling can be helpful, but hacking and controlling enemies can save your soldiers a great deal of hassle. Firstly, you'll have an extra unit to help you fight, but more importantly if you move the controlled mech unit close to the front of your units, the enemies will target the mech units, rather than your soldiers. Also, hacking the security lights (whatever they're called) can grant nice bonuses (increased supply income or increased hacking skill, for example).
-Say hello to my little friend! Grenades, grenade launcher, explosives in general can be VERY helpful. Not only do they have an area of effect with a 100% chance to hit and armour shredding, but they will also destroy an enemy's cover, turning a 34% chance shot into a 72% flanked shot with 40% crit chance. Very useful.

Less important tips
-Don't forget to gear up well! I did rush a bit too much when I started (as i advised against above) and found myself with low level gear and wasn't hitting hard enough. I would apply this mainly to weapons, moreso than armour (I completed this game using only tier 2 armour, but tier 3 weapons).
- Don't ignore the avatar meter, but don't focus on it. I didn't find the avatar meter too much of a problem, I only had to take down one avatar factory to buy me a small amount of time.
-Engineers are probably more important than scientists at first. Engineers should always be working on something. Usually assigning an engineer to a facility (e.g. resistance comm) is much cheaper and more efficient than upgrading.

General tips - these apply to xcom 2 in general, rather than commander ironman but are very useful for this achievement.
-Try not to let your units die. There's a high chance you're going to lose a unit or 2 early on but hopefully only rookies. This is OK. But once they get a class and have been promoted a few times they become MUCH stronger and losing a high-ranked unit can be very detrimental.
-Linking in with the above point, try to get a solider promoted as high as possible as quick as possible. Some of the higher-promotion abilities can be a real game-changer.
-Shredder! Always have this ability with your grenadier, the armour of some enemies can really sponge some of your shots, shredder will take away 1-3 (depending on your grenadier's weapon tier) points of armour. Also, when possible, I'd recommend attacking first with your grenadier, lowering armour, then using your other units to hit with higher damage.
-Executed! The weapon mod 'Repeater' offers 5/10/15% chance to insta-kill and can be very useful for the last mission. I'm going to assume you know what the last mission is and what you have to do (if not look away now or spoiler!) - kill the 3 avatars while getting swarmed with aliens/ADVENT. Ideally you'll want a superior repeater with 15% chance, but an advanced (10%) may do. Equip the mod to as many soldiers as you can (assuming you have more than 1, i only actually had 1 when i did it and that did enough), this means you have a chance to instantly kill an avatar, assuming you can get close enough. If you don't instantly kill the avatar you can always reload and try again, if you feel you need to.

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: