XCOM 2

Found this on StackExchange, props to Isuka:
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Of note is that this only applies to the final mission, you can happily upgrade throughout the game, just make sure your squad going into the final mission is downgraded. (Tip: Psionics/Dominate is AWESOME).

Only Tier 0 items:
- Assault Rifle, Pistol, Shotgun, Cannon, Sniper Rifle, Sword, Gremlin Mark I, Psi Amp, Grenade Launcher
- Nanofiber Vest
- Medikit
- Frag Grenade
- Flashbang
- Smoke Grenade
- PCS Upgrades
- Weapon Mods

Exceptionally, the following upgrades are allowed: (since you can't downgrade)
- Alien/Plasma Grenade (Permanent upgrade to Frag Grenade)
- Nano Medikit (Permanent upgrade to Medikit)
- Smoke Bomb (Permanent upgrade to Smoke Grenade)

Forbidden Items include (not comprehensive - ANYTHING not in the list above may be bad):
- Skulljack
- Battle Scanner, Mindshield, Mimic Beacon
- ANY Special Ammo (Talon, Venom, Dragon, AP, etc.)
- ANY Special Armor (EXO Suit, Warden armor etc.)
- ANY Special Grenade (Gas, EMP, Acid Grenade/Bomb, etc.)
- ANY better weapon (Advanced Grenade Launcher, Arc Blade, etc.)

In essence, the game checks the inventories of all soldiers and makes sure that you only have Tier 0 items, with the exception of the three permanent upgrades (Alien Grenade, NanoMedikit, Smoke Bomb) and any drops you got throughout the mission.

The other solution provides a great breakdown of what constitutes the requirements of this achievement, and I recommend it to everyone, but I wanted to add some tips for how to win the mission.

As stated in the other solution, your soldiers can only bring starting-tier weapons, armor, and miscellaneous gear like the gremlin. The most important exceptions are the plasma grenade, which you learn to make in the proving grounds after completing the muton autopsy, and the nanomedkit, which is obtained by completing the viper autopsy and then the Battlefield Medicine project in the proving grounds. Another research you can benefit from is Modular Weapons, which lets you apply 1 upgrade/mod to the starting-tier primary weapons. You can apply PCSs to your soldiers. If you want to waste a utility slot on smoke grenades/bombs, flashbang grenades, and nanofiber vests, go for it, but IMO you should only be carrying plasma grenades and maybe 1 medkit.

If you can get the "Armed to the Teeth" continent bonus, get it. That gives you a second mod slot for your weapons and it is definitely compatible with the achievement requirements.

You can earn this achievement in Rookie difficulty. In my Commander playthrough, the Shadow Chamber told me there were 31 enemies in the final mission, as opposed to 26 in my Rookie playthrough. If you want to switch difficulties midgame, I would suggest doing it before you perform the Avatar autopsy. You may be able to switch it at some point after, but I would do it then, just to be safe. If the Shadow Chamber tells you you have more than 26 enemies, you probably aren't on Rookie difficulty. Note that this only counts the enemies who are already in the base when you get there; not the Avatars and their minions who enter through the PSI gates at the end.

I recommend bringing three well-trained PSI operatives on the mission, since they are less weapons-dependent. For my playthrough, I brought a grenadier with 3 plasmas, a hybrid healing/hacking specialist, a ranger, and 3 PSI operatives. As much as I love the sharpshooter class, I feel like the level design hampers their usefulness, and you get more benefit from a third PSI soldier.

Once the mission starts, go SLOWLY through the first part. There is no turn counter and there are no reinforcements, so there's no reason to rush it. Avoid using your grenades, if possible, and don't use up your limited-use abilities like Dominate. You'll need those for the second part. The commander's mind control is good to use, because it recharges. Save often!

The last section of the first part is the one where there are small earth-like habitat rooms. When you kill the last enemy, you will immediately begin part two of this mission, without any chance to reload or heal or anything, so When you're on the last enemy in this section and know you're about to inflict the killing blow, let your other soldiers reload first. If you can throw a stasis on him, do it to give all of your other abilities another turn to recharge.

Once the second part of the battle begins, in the large arena with the PSI gates, unleash all of your Mind Control, Dominate, and other abilities. If you bring 3 well-trained PSI operatives like I recommend, then with their Dominate plus the commander's Mind Control, you can have 4 aliens helping your 6 soldiers for this part. With all that help, who needs Tygan? Try to use every action you can on killing avatars and only deal with their minions if you don't have a shot on an avatar. Remember that they teleport every time they take damage, so if you have soldiers who don't have a shot, don't move them; let a soldier who does have a shot use it, and maybe the avatar will teleport into range of one of the others.

If all goes well and you kill the third avatar, you won't get the satisfaction of seeing and hearing the achievement unlock until some cutscenes play. It unlocked for me while an elder was saying "You are not ready" to the commander.

Good luck!

In addition to the excellent solutions by labtoy and Lt Davo I'd like to add some more tips to beat the final mission itself.

First of all, Rookie difficulty is key. Enemies are far more likely to miss their shots at you, and it feels as if your shots hit them way easier especially at low hit percentages. This will also avoid save scumming, because you simply won't need it.
Make sure to give all of your weapons an upgrade, such as the hair trigger or laser sight. Extended clips or auto-loaders are not recommended, as you will have more than enough time to reload.

Once you start the battle, move slowly. Only use the first action to move (the fields that are highlighted in blue, not yellow) and put your unit on overwatch at the end of the turn. As the mission is not timed, you can definitely take your time here.

Even though all your soldiers will have the standard gear, your Commander's Avatar will have a powerful gun, and even more important, PSI attacks. Make sure to use his gun, the lance, and the mind control on enemies that can't be killed by other soldiers, or to simply keep them occupied for one more turn so that the other soldiers can kill them later. In addition, if you gave your grenadiers the "rain of bullets" ability, use it frequently as well, as this guarantees a hit with a decent amount of damage dealt.

By moving forward slowly, you should only encounter one enemy group at a time. As soon as you discover the group, they will become alerted and disperse. Here's the important part: Directly following the discovery, use your Commander's Avatar to hit the group of enemies with his dimensional rift ability. It's radius is so wide that you should be able to hit the entire group of enemies most of the time, either killing them right away or significantly weakening them. This will make it very easy for your remaining soldiers to kill the weakened enemies off one by one. As an example - you will encounter a group of six or seven MECs early on; I was able to kill all of them but one with a single dimensional rift attack.

As soon as the group of enemies is killed, do not continue moving forward. Have all your soldiers reload your weapons and overwatch, until your dimensional rift attack is available again, which should take five turns (you'll have any other ability such as "rain of bullets" available again as well). Once that is done, proceed forward.

At later waves, moving forward after encountering a group of enemies might trigger a second group, so better stay put and keep your soldiers in cover until all enemies are dealt with.

As soon as you enter the final stage, it's all about killing the three avatars, you can almost ignore the other enemies. New enemies will spawn constantly, so you won't be able to keep up, anyway. Keep in mind though that the Codex can disable your weapons, forcing you to not only move your soldiers but also to reload, potentially wasting an entire turn, so you might want to take them out if the opportunity exists.
Again, the Commander Avatar's abilities are the best weapons here, so use them as much as possible.

If you did not train any rookies to become PSI operatives, do not worry. It will help you beat the mission just as Lt Davo mentions, but the mission is also very much doable with the Commander's Avatar being the only one with PSI abilities.

Go get 'em, Commander!

not a solution, but an addendum to the other solutions in regards to the Shen's Last Gift and War of the Chosen DLCs:

You can earn this achievement using both these DLCs.

The rules are still the same:

- Any DLC soldier must be sent into battle with their basic starter gear

SPARK:
Spark Armor
Heavy Autocannon
SPARK BIT
Rocket Launcher

Reaper:
Reaper Armor
Vektor Rifle

Skirmisher:
Skirmisher Armor
Kal-7 Bullpup
Ripjack

Templar:
Templar Armor
Shard Gauntlets
Autopistol

- If you happened to buy the accessory perk for any of the WotC soldiers, you are limited to the same items. NOTE: if you see a * next to the listed item, there is an upgraded version of it. The upgraded versions are okay to take into the final battle, they will not cancel the achievement.

Frag Grenade* (upgrades to Plasma Grenade)
Smoke Bomb*
Medikit* (upgrades to Nanomedikit)
Flashbang Grenade
Nanoscale Vest*

- All PCS are okay to use

- All weapon upgrades are okay to use

- All weapon research upgrades are okay (+1 weapon damage, +1 upgrade slot)

- All GTS soldier perks are okay

- Can be done on any difficulty

Tips:

obviously set the difficulty to rookie. this will make the 2nd part of the final mission tolerable.

the final mission takes place immediately after completing the Advent Network Tower mission. It is broken up into 2 parts. The first part is straightforward and not too bad. Make sure you save all your accessory weapons and one time use psi skills for part 2. Part 2 is where you must kill 3 avatars while portals keep farting out random groups of enemies at you. Mind control is very useful (especially if you can MC a gatekeeper) for distracting enemies during this 2nd part.

I HIGHLY ADVISE you defeat all 3 chosen before you do the Advent Network Tower mission. If you do not, the chosen will appear in the first half of the final mission to complicate things.

save scumming on the 2nd part is useful. not for rerolling attacks, but in the off chance you hit an avatar and he teleports so far away no one can reach him to shoot.

SPARK: would only recommend taking one into final battle for Matter Over Mind achievement if you don't have it. The SPARK's wimpy starter gear makes it a poor choice going for Who Needs Tygan? SPARK weapons do not have breakthrough techs so you're stuck with a low damage weapon with one upgrade slot despite having an OP skill like overdrive and native shred properties on its weapon.

Reaper: have not used this class for the final mission. Unsure if they can remain hidden against avatars. But if they can be stealthy, I can see them being very useful in the final battle in 2 ways:

1. claymore use with shrapnel, highlands, and homing mine perks. while hidden, toss a homing mine on an avatar. Use sting skill or have someone else shoot to keep reaper hidden and do the same to next avatar.

2. blood trail (maybe), needle (not needed on rookie difficulty as avatars have no armor), banish skills. superior extended mag weapon upgrade. honestly, sneaking around in cover on rookie and using banish with maximum magazine should be an easy no fuss method of killing an avatar. problem then is if its the first or second avatar, the reaper will be left open to any nearby enemies.

Skirmisher: their basic weapon is crappy and weak. However, you can take advantage of the fact they can shoot twice per turn as long as they don't move. Slap a superior repeater and an extended mag or autoloader on their gun, and if you're lucky to have the inside knowledge WotC skirmisher order or region perk, the Skirmisher will have a 20% chance to do an instant kill. Somehow I've had lots of luck one shotting avatars this way. Outside of this, I don't see how they're useful for Why Needs Tygon? They'll be doing an average of 1 damage if they manage to hit otherwise.

Templar: With maximum focus, they will be the heaviest hitting units with basic weapons 2nd only to the commander's avatar. Make sure to buy the channel perk. This will make it very easy to keep them at max focus. I'm not sure if the deep focus perk helps improve damage, but it also gives one extra focus point. My Templar was doing 7 damage minimum with rend.

Psi Operatives become more useful with a Templar around if they have the inspire skill. You can chain Templar attacks in one turn to take out many weak or one tough enemy.

I found myself following this guide: https://steamcommunity.com/sharedfiles/filedetails/?id=65526...

In a TL:DR version, armor and all primary and secondary weapons must be the base version. Medkits/Nanomedkits and Frags/Plasma grenades are also allowed. Smoke and Flashbangs are probably OK, but generally not recommended.

SPARKs are allowed, but like regular soldiers must be using base level weapons and armor. Their equipped heavy weapon must be the Rocket Launcher.

DESPITE WHAT HAS BEEN SUGGESTED IN SOME GUIDES, any/all weapon mods and PCSes are allowed, so go hog wild on those. This makes sense given they aren't acquired through research.

It is known you cannot use the Alien Hunters weapons.


From there, the game plan tends to be much like other guides have suggested. Play on recruit difficulty. You're best bet is a team that is heavy on fully trained Psi Ops as they tend to have a lot of bonus abilities that aid in survival, crowd control, and single target burst all rolled into a single unit.

My team was as follows:

SPARK (mainly hoping to kill an avatar with it for the achievement, replace with what you'd prefer if you follow me)
Adaptive Aim, Strike, Intimidate, Repair, Hunter, Nova
(base game, so no weapon mods)

Specalist
Nanomedkit
PCS: Superior Focus (+28) (wanted to avoid mind controls as much as possible on this character)
Weapon Upgrades: Superior Repeater, Superior Auto-Loader
Medical, Haywire, Field Medic, Covering Fire, Guardian (was not max rank, but I would have gone with the revive everyone ability)

4 fully trained Psi Operatives all with Plasmas

3 with Superior Conditioning (+4) 1 with Superior Agility (+33)

Weapons:

(x2) - Advanced Repeater, Superior Stock
Repeater, Superior Hair Trigger
Superior Hair Trigger, Advanced Auto-Loader