Harley isnt too bad but she has like 12 bad match ups. Which characters give her tough time? Characters with great zoning or footsies so like Sinestro, Deathstroke, Black Adam, Superman, Aquaman, Martian Manhunter, Batgirl, Killer Frost, and list goes on.
The zoning game here strong so Sinestro would give you most trouble with her. From far range she at huge disadvantage vs these characters cause they got good zoning and/or footsies. It doesn't help up close she also not that great either when her footies are pretty much ass
I main a top 5 character(Superman) and i'll tell you this MMH will be a pain. NRS decided to make the teleport safe. And it not a MB either.
Though go with who you like just know the uphill battles with her. I would recommend Killer Frost though not sure if you would like her but shes got a sick vortex game.
d1- This will be the option you use to apply pressure and mix ups. Combine with trait makes it deadly making it a low-overhead mix up. This really only option your opponent will fear the most since it a low and with trait it launches. He does decent damage