Starting position:

Your Party is at the bottom of the stairs (30' from the landing at the top and the Animated Armor) in this marching order:
Paladin
Barbarian
Thief
Druid
Cleric
Warlock

Initiative order:
Barbarian
Animated Armor
Cleric
Thug
Druid
Witches (3)
Paladin
BBEG
Thief
Warlock

Of note, your Barbarian, Thief and Paladin all have been "touched by the mist" and therefore the BBEG has advantage on ability checks and attack rolls against these PCs. Similarly, such characters have disadvantage on any saving throw made against BBEG.

The bad guys in the upper room will ready actions if no party members have moved the Animated Armor out of the way, opened the door, and stepped into the room.

Choose your spells and actions. What will you do and WHY?

(My party was TPK-ed. We did not know how to stop the ritual, we really struggled to get up the stairs and past the animated armor, and then the BBEG kissed us into spectators. We got SPANKED.)