How about asking him to set a trap or something? Like, let's say, this guy makes the trap, then you or Maria go ahead and set it up. Wouldn't that technically count as him killing the monster?

Also, I'd like to recommend fire. Burn it to a crisp and eat it with ketchup afterwards. Maybe set a trap with oil and a lighter.

Acid is also a good option. Insecticide just couldn't do it tbh. Even drowning the thing in insecticide might not be enough. Droen it in acid and all you'd have would be acid.