Spells known/prepared is essential knowledge. I don't want to recommend something that you just can't do, e.g. tell you to spam Vrocks and conjured animals up the stairs for the first five rounds or so, if it turns out that the Celestialock doesn't know Summon Greater Demon and the Moon Druid doesn't know Conjure Animal. (Vrock is first up the stairs, Stuns, and then next round releases spores. Animals charge up there after the Stun to take advantage on the opening. PCs at the bottom can all just focus on killing the Animated Armor easily.)
BTW, wow, Goblin Thief? What a waste of a racial feature! Should have been a Goblin Bard.