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.

The barbarian PAM build so ditching his staff and grappling probably never even cross his mind. Certainly not until one round of misses passes by trying to futilely hit the armor.

So maybe after the first round of attacking the armor if the team realizes it has high AC they might change to having the barbarian grapple it and shove it prone so it has zero movement.

The rest of the party charges up. Paladin should engage Thug. Druid should cast summon animals (giant owls), cleric should cast a spirit guardian upcasted, warlock no clue what spells he has access to should cast something useful.

If the BBEG kiss is successful I think maybe a dispel magic or calm emotions should be dropped.

Giant owls should fly in and start beating on the BBEG, and witches. The fight should go well for the group once the Thug and witches are taken care of. The only thing is that the barbarian will need to outlast the animated armor with his rage and hps.