Earlier, after I gained the upper hand and killed them one by one, Grand, who advised me to buy some time, generated a skill that will be of use to us in the current situation and for the future.
A skill similar to Eric's ability, discerning something beyond common knowledge of magical abilities.
The Mind Reader skill allows Grand to know what the targeted individual is thinking.
After Grand successfully created the skill, I manipulated the conversation and directed it toward letting the elf prince think of his home.
I left Grand to analyze and use the new skill to obtain information from his mind as he argued with me.
Back at the current date, Grand had finally gotten the information we wanted. In order to stop their plans to attack the village, which I tried rebuilding, attacking the ones who'll pose a threat to us in the future is one of the best moves I could think of.
[Yet you forgot to ask about their stealth ability.]