While promoting the Warner Bros. film on BBC Radio 1, she and co-star Will Smith were challenged to a game of "Playground Insults," in which the goal is to make the other laugh.
What began shakily enough with a crack about Smith's ears quickly escalated as Robbie opened the door to a comment about her mother before quickly warning, "Don't talk about my mom!"
"This is the problem - we know too much about each other," said Smith, who also starred opposite Robbie in "Focus."
Seemingly disarmed, the Deadshot actor became a target for jabs about "The Fresh Prince of Bel-Air," "Big Willie" and, of course, his appearance.
"You were the worst-looking cast member in 'I Am Legend,'" Robbie said.
"And it was me, zombies and a dog," Smith replied, "and I was the worst one."
However, he finally did land a blow about Robbie's mother, only for her to quickly return fire. Watch the video below.