How to predict what will damage or heal an opponent?

On several occasions our actions healed our enemies, where there was no apparent clue that this would happen. Surely there is such a clue, and we just don't see it. I'm here to ask if anyone can enlighten us.

How can you tell if your action will heal instead of hurt the opponent?

Here's an example situation that highly confused me. We got into a fight with a Goblin named Kadraskaz, and all of the following seemed to heal him:

• Pyrokenetics spells such as Burning Blaze (Fire);
• Geomancy spells such as Boulder Bash (Earth) and Magical Poison Arrow (Poison);
• Aerotheurgy spells such as Headvice (Air);
• Hitting him with a staff that does Water damage;
• The Earth-damage from a Piercing weapon (the piercing damage was normal);

This was the enemy's status at the time:

The fact that I started my question saying we had "no clue" that the enemy would be healed was because I figured that "Resistance" would worst-case reduce damage to zero; but perhaps "Resistance" from a certain point (100?) onward should be interpreted as "Will actually heal the guy"? In my example above that would mean only physical damage was useful, right?