By 'knows where to hit someone where it hurts' I mean someone who uses another person's weaknesses and feelings to their advantage. Say if their opponent had low self esteem about a certain part of their body the person would make sure to bring it up in the argument to weaken their opponent.
Gaslighting is a tactic in which a person or entity, in order to gain more power, makes a victim question their reality. It works much better than you may think. Anyone is susceptible to gaslighting, and it is a common technique of abusers, dictators, narcissists, and cult leaders. It is done slowly, so the victim doesn't realize how much they've been brainwashed.
and a gaslighter is:
One who engages in the psychological tactic of gaslighting.
If online, you can use the word "griefer":
A griefer is a player in a multiplayer video game who deliberately irritates and angers other players within the game, often using aspects of the game in unintended ways. A griefer derives pleasure primarily or exclusively from the act of annoying other users, and as such is a particular nuisance in online gaming communities, since griefers often cannot be deterred by penalties related to in-game goals.