Quote Originally Posted by Gignere View Post
This is a hard setup for the party in question. There is no flipping way to guess that the animated armor is susceptible to dispel magic unless some kind of hint was dropped by the DM or the players cheated.
It's standard for animated objects. Totally plausible for PCs to know. Would be more surprising if it turned out NOT to be vulnerable. In any case it's simple enough to kill before entering the room--how you kill it basically doesn't matter. It can't stand up to six 9th level PCs and their summons.