Do Gatekeepers have any weaknesses that can be exploited?

Solution 1:

EMP Grenades/Bombs

So I just learned from Are Gatekeepers vulnerable to EMP Bombs? that Gatekeepers can actually be heavily damaged from EMP grenades/bombs, and can even have a chance to be stunned by them for 2 turns!

I tested and confirmed this myself. In the following screenshot, I used an EMP Bomb on an undamaged Gatekeeper, and it took something like 12 damage, and was also stunned:

Which is weird, because you can't hack a Gatekeeper, so who knew it had robotic components that could be damaged by an EMP?

Flashbangs

I've heard that Flashbangs (or being disoriented) can lower a target's Will stat, which would make it more vulnerable to mind control. I decided to test this out too.

The following is the Will stat of an undamaged Gatekeeper versus a fully upgraded (i.e. has all abilities and fully upgraded Alien Psi Amplifier) Psi Operative using Dominate:

The target's Will is 110%, which reduces my chance to Dominate to 80%. After hitting the Gatekeeper with an EMP Bomb, followed by a Flashbang (so that it's also Disoriented), I checked the chances again, and the Gatekeeper's Will was still 110%.

I've also heard that Flashbangs (and being disoriented) can disable some of the Gatekeeper's abilities, but I haven't tested this out yet, and I'm not sure which abilities exactly would be disabled.

Solution 2:

When they open (for a melee attack or mass ressurection) they lose most of their armour for one hit.

But better are psi attacks, the best is Dominate as you get them for the rest of the mission as it doesn't end like Haywire Protocol.

Null Lance will do a lot of damage, can't miss, and because they are 4 tiles it makes it easier to hit them and something else.

The single target psi attack also does about as much as a weapon shot would if they had no armour.

I believe Insanity would work on them but haven't tried.

Psi storm can rupture them making them take more damage

in the last mission with the avatar he can mind control them for a few turns, if you want to not have them mess up your ranks when that ends use statis when your avatars mind control is on T-1 cooldown and re control next turn

for other people you can equip AP rounds which peirce 5 armour, on a sniper or shotgun with laser sight this can do a lot.

In WAR suits the shred blast, plasma shot, and blaster bomb all remove a lot of armour.

Solution 3:

From what I've seen Gatekeepers tend to travel with some fairly weak companions, so you can focus fire on them pretty heavily and largely neglect the rest of the pod (or kill the remainder with just a few actions). By the time you're encountering Gatekeepers you should hopefully have or be close to having plasma weapons.

There are a few options available to you:

Burst it down

With a psi soldier and some shredding ability, they're pretty easy to deal with in two turns:

• First turn - focus on shredding armor. A grenadier with Salvo is great here, because you can fire a grenade, then still shoot with Shredder ammo, or even fire a second grenade. Acid Bombs are great for shredding armor, but other grenades help too. After the rest of your squad has acted, have your Psi soldier use Stasis on it.
• Second turn - pile on damage. Sniper shots, Chain Fire, more grenades with Salvo, anything you can do. Your Psi Operative's Null Lance is very effective here, or you can try to apply Rupture so it takes extra damage. If it looks like you might somehow fail to kill it on the second turn, you can use Insanity on it. If you hit, you might get lucky and panic or Mind Control it. Even disorienting it will make it harder for it to run up and melee you, or use its powerful beam attack. (It may even disable its psychic attack, but I'm not 100% on that.)

As long as you don't miss too many shots, and you're not run out of grenades at this point, this is a pretty safe strategy.

Take it over

Despite having a powerful psychic attack, Gatekeepers are not that difficult to Dominate (roughly on par with Archons). Assuming your Psi Operative has been keeping busy you'll probably have a 70% chance to take it over. This is definitely a riskier strategy, because if it fails, you just lost your best option (Stasis) for keeping the Gatekeeper under control this turn. When it works, though, you get what is probably the best creature in the game.

Run away and spread out

If for some reason you can't try the above strategies - say, you don't have a psi soldier for some reason - you can try to minimize damage. Gatekeepers have three attacks:

1. A single-target beam attack for heavy damage.
2. A melee attack for medium-heavy damage which heals the Gatekeeper.
3. A psychic AOE blast (roughly 4-5 tiles radius) for medium damage.

If you have some room to run around, you can kite the Gatekeeper while taking shots. You want to be sure your soldiers aren't grouped too tightly. The ideal outcome of each turn is for the Gatekeeper to use their psychic attack on just one or two soldiers, because it opens their shell, reducing their armor for the first shot of your turn. You definitely don't want to leave anyone in range of the melee attack, or you're going to prolong the fight further.

This is definitely the riskiest strategy. It requires you to leave the Gatekeeper alive for a long time, hoping your cover is good enough to avoid getting shot. And since Gatekeepers destroy cover they move through, you're not going to have many good options to go back the way you came after the fight is over.

Solution 4:

You could stall a Gatekeeper with a psi soldier stasis bubble for a turn, but that also renders the Gatekeeper immune to attacks for that turn.

Remember that you can attack first, and then use stasis as the last thing you do. I've dealt with many enemies just by dealing damage over multiple turns, preventing them from fighting back with stasis - sometimes chain-stasis from multiple psi units.