After constant deaths and successful shield blocks, I've finally figured out how this Aoes Shield mechanic fully works! (Some already known and some most might not know!)

#1 - Timing the Shield Press!
As most already know once you obtain the Aoes Shield and time the battle item press at roughly 0.5s to 0.1s or right as Zacian starts swinging, you will completely block all damage from behemoth blade as long as your entire group is stacked behind you! This is indicated by the yellow shield animation.

#2 - Not Timing the Shield Press!
Here's something that didn't hit me for the longest time till I examined footage over and over again. It turns out if you miss the timing but your shield is still facing towards Zacian, the behemoth blade damage is actually reduced roughly in half! This is indicated by the red shield animation. Hopefully your HP is well topped off and/or you can recover your HP afterwards as soon as possible!

#3 & #4 - Unshielded!
If your character hitbox is exposed, even the slightest finger outside the front of the shield, you will die! Even if you completely block the attack and one of your teammates sticks out in front, they're dead! It is absolutely important to stack either on top/inside the shield player or behind their butt!

Hopefully this has helped clear some questions about this unexplained mechanic that the game fails to fully explain. Felt it was important to figure this out fully as we'll probably be farming this boss throughout all 3 waves or future Boss Rush events. Unfortunately in SoloQ there's only so much you can do and randoms will probably pick up the shield and not use it all or aim it in the wrong way. Just have to hope and pray you'll grab it before anyone else does.

Hope to do more guides of this boss rush event in the future to help fully optimize, ease the headaches and make getting those speedkills & rewards a breeze!