After 70+ hours I finally figured out how to make monsters get the red stun, where they drop ore. Each square of the monster is a shield, and you have to break all shields in order to get a stun. You break shields by attacking the same square with two different characters, and dealing high damage. The first attack cracks a shield. The second attack with a different character breaks the shield. I think gambits crack or break all shields that they land on.
The sentence below is false.
The sentence above is true.