Tactical Challenge #4

[Unoriginal]

I'm confused. Do you want to DM this encounter?

First thing to point out is that with the wording the BBEG chose, the party can safely ignore the plate armor as long as they can 1) get in initiative order b) all of them except the last PC in the initiative order prepare the "move through the door as soon as it's opened/as soon as X teammate when through" reaction c) the last one on initiative order open the door then go through it on their turn.

The animated plate guard the door, it won't do anything if all the PCs cross it before its turn.