This does seem strange to me as well. The witch should know the Hou family fights in actual wars, so even if she was just measuring Yuuri's resolve, this indeed seems too cheap. You can't measure a warrior's resolve like this. It should be done through nefarious means, where the warrior can't rely on his best talents and skills. Like poisoning in secret, since poison was mentioned here. Isn't poison something that ought to belong to a witch family anyway? Poison someone close to the warrior and see how he will react. Of course the warrior might just attack in a straightforward manner in retaliation, knowing the identity of the enemy, so it's not necessarily a better option. Dependency on antidote might still work. Anyway, the witch (mafia) should work using their own best methods, not try to attack using the opponent's best methods.
Maybe the witch mafia has had so easy for so many years that they don't genuinely know what to do if the opponent knows how to fight back at all.