This is situational as will all of these be,but:
In HU you can look to tilt a player. This is their weakness,they are tilt prone.I know this is a little off to the side of what OP had in mind but it stills constitutes a weakness nonetheless.
Things like 3 betting then c-betting any of their raises will tilt them (this has to be done with skill or you're going to land yourself in it)
Raising every single button 3x until they show resistance or just plain tilt off stack (again skillfully), follow it with c-bets etc. Some people just can't handle the constant pressure and they just go off on one.
My personal favourites though have to be:
Typing "n1" in the chatbox everytime you win a pot or you beat them at showdown, also especially in big pots.
Typing "OK" everytime you take a big pot. Try to make it look like your foreign as hell.
Just work out how to tilt them and do it. Turn off their chat as well so they can't fight back, real mature