[Endoperez]

If you have at least two spells, the masters' skills are increased. It might not show up.

Body Ethereal is a good spell. Casting it isn't a bad choice. It makes powerful units, like Elephants, incredible. However, casting it on archers is a poor choice. The AI isn't clever enough to make that distinction. I presume the Theurg kept casting it on archers or some other nearby units.

It might also be the case of Body Ethereal having AoE of 1 square, or 3 humans, while e.g. Mind Burn only targets 1 person. As such, the spellcasting AI might think that targeting 3 people is better than targeting just 1.