Jacob is not weak, but their vulnerability was sample mean, a killer has to hide his presence, if Jacob had to hide his magic, when it is revealed it becomes vulnerable.
What did I mean is, when someone discovers the plan of the killer he becomes weak and that's what Jacob listens.
Jacob uses spatial or mental magic?
It seems that the spoiler said he hides the presence of people in the conscious of the enemy, if every blow that Natsu and Lucy err or deviate hit someone guild, because they are all there, but only Jacob is able to see and when they won members will be...