xcom-2
The Will stat helps soldiers resist negative mental status effects from psi attacks such as mind control.
However, there are some psi attacks that merely do damage, such as the AOE psi attack of the Gatekeeper.
Does the Will stat help reduce the amount of damage taken from these attacks? If not, is there anything that can help protect our soldiers against direct damage psi attacks? Armor certainly doesn't help, since psi attacks bypass armor.
According to XCOM Wiki:
The success of psi attacks is only affected by the operative's Psi stat - which is increased by using upgraded Psi Amps - and not Will. Will is only a factor when defending against psi attacks, and improving Will (such as through a Personal Combat Sim) will not increase the operative's Psi rating. In short: Psi = Offense, Will = Defense.
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?
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.
Best Answer
Don't know about anything that can help with psi attacks. Both Mind Shield and Solace are helpful against all psi attacks other than attacks that do direct damage.
The psi attack amount is more related to the enemy's Psi Offense Rating.
In this case offense is your best defense. Use flashbangs or other ways to disorient enemies. This will restrict all their special abilities.