Or, Marcy has a Restraining Bolt placed on her, and Lisa takes this as an opportunity to tease her without being scalded.
Lisa is jealous of Marcy and her power, and barely even cares if Marcy kills her as long as it takes that stuck-up witch down a peg in the process.
Lisa is secretly a Death Seeker who is trying to goad Marcy into killing her.
Lisa has too much Pride, she thinks bullying someone with ridiculous powers and standing on the edge of being annihilated would make her look like a badass in the eyes of her peers, when in fact that is not the case.
Marcy uses her powers to terrorize everyone and keep them afraid of her.
Marcy is a super-powered bully, and Lisa the only one who dares stand up to her.
Alternatively: Lisa and her Girl Posse suck up to Marcy because her powers could come in handy in the future.
Lisa picks on someone they know is too vulnerable and powerless to fight back.
Lisa sucks up to a person way less powerful than she is.
Lisa, after having the power discrepancy pointed out, decides not to tease or antagonize Marcy anymore.
Alternatively: Lisa and her crew pick on Marcy despite her witchery being common knowledge, but later we find out Marcy is joining their sorority and hazing is par for the course for everyone. Marcy knows she is not getting any special treatment.
Or: Lisa has heard the rumors, but believes that Marcy is either delusional or faking it. When confronted with a display of actual power from Marcy, Lisa backs down rather meekly.
Or: At first it seems Marcy can easily kill the bully, but later it reveals that Lisa is just as powerful witch as Marcy, if not more, so she has no reason to be afraid.
Or: They're good enough friends that everyone knows the teasing isn't serious.
...At least, not to her face. But playing pranks on her? Fair game.
Lisa and her crew use this opportunity to really hurt Marcy because she's a witch, under the guise of common hazing.
Marcy is an actual full-sized dragon. Lisa insists on throwing rocks at her anyway.
Alternately: Every time Marcy and Lisa are in the same room, everyone else runs for cover.
Lisa refuses to openly harass Marcy, but she and her Girl Posse secretly play nasty pratical jokes on Marcy. Marcy figures this out and confronts them; Lisa immediately backs off, shocking her friends by apologizing. Then they start picking on Iris, Marcy's much-less powerful friend... and when Marcy finds out, she is through the roof with rage.
Alternatively: Lisa and crew pick on Marcy despite knowing she's a powerful witch, but they're just hazing Marcy like their sorority normally does to their new entrants. Then they drug her, take embarrassing pictures of her, blackmail her, then release the pics to the Internet anyways to "put that prissy witch in her place". Marcy snaps... only for Lisa and crew to reveal they had superpowers all along and can shrug off anything Marcy could throw at them. But all this was a scheme to get back at Marcy's boyfriend for dumping Lisa, even though they know he is a Bully Hunter with Psychic Powers they are not immune to.
Only people who have a chance of surviving Marcy's retribution pick on her.
Alternatively, the story dispenses with bullies altogether.
Lampshaded: "Lisa, you know she could barbecue us any time she wants, right?"
Invoked: Lance, Lisa's crush, double dog dares Lisa to pick on Marcy.
Exploited: Lisa picks on Marcey, knowing that she will end up getting hurt, and will subsequently sue her for a large sum of money.
"Are you insane? We're not picking on anyone we can't beat up if they decide to fight back. We're bullies, not idiots."
The first time Lisa starts mocking Marcy, she gaves a splendid beatdown to the bully and tells her that, if she dares to speak with her in that tone again, she will kill her.
Discussed: "There go Lisa and her gang again, picking on Marcy. Are you sure they know she's a witch?"
Conversed: "The bullies on these urban fantasy show sure are dumb; they've already seen Marcy wipe out the gymnasium."
Lisa is full of self-loathing, but cannot bring herself to commit suicide, so she's provoking Marcy in an attempt to get Marcy to kill her. Or she really is that stupid and bitchy and there are other situations where she shows off her lack of survival instinct to indulge her pettiness (like walking into traffic to yell at someone she thought snubbed her).
Marcy just kills Lisa off when she goes too far. Then the local witch hunters show up out for Marcy's blood, viewing her as nothing more than another murderer they need to bring to justice.
Marcy breaks and tries to kill Lisa, but manages to control herself and redirects the fireball into a nearby tree. Afterwards, Lisa and Marcy reconcile and become friends. Marcy soon discovers that she can't produce another fireball that big without duplicating the situation, resulting in Lisa becoming her sidekick in order to intentionally tease her into full power when the need arises.
Marcy is well aware that while she could kill Lisa any time she wanted to, that doesn't make it right. So she decides to look for a more reasonable solution to the problem. Until she finds one, however, she'll have to put up with Lisa's teasing.
Played For Laughs: A recurring extra ends up getting Amusing Injuries when trying to beat up Marcy for being a witch. This turns into a Running Gag.
Played For Drama: Marcy is this closeto snapping and misusing her powers, and Lisa and her flunkies just keep dancing on her last nerve...
Go back to Bullying a Dragon? Even though you know it hates your relentless clicking?