What types of enethe attacks can shred armor

• The attack is only triggered by enemy movement. Moving on your turn will not trigger Bladestorm (unless your move causes an enemy to move, such as a burrowing Chryssalid).
• No, only the movement will trigger Bladestorm. I've had an Andremedon move past my soldier (triggering Bladestorm), then in a separate action, make a melee attack. The attack was not responded to by my Ranger.
• They happen immediately, usually before the enemy gets to take their action. This is very useful against, e.g. Stun Lancers, which will move into attack range but die before they can actually make an attack. Some enemies, such as Faceless, have a melee range larger than the Ranger's, and don't actually have to move close enough to trigger Bladestorm to attack. (It seems there may have been a recent patch affecting this; it's possible for Bladestorm not to trigger until after the enemy gets to attack.)

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.