Unorigional - The Animated Armor will attack anyone in melee range, and stand in front of the door where no PC's can pass through his space to access the door. It has a INT of 9 and understands "guard the door" better than all the 8 INT PC's would due to it's superior Int. It will act just as tactically as they would.

Val: The only time limit is 10 rounds to complete the ritual, but the party has no knowledge of this time limit. The BBEG must maintain the ritual every round, but there is nothing RAW that requires them to stay in the room (but RAI - I'd assume they would).