system
Given the following post, respond as if you're commenting on Reddit:
human
[DC] Why is Harley Quinn a good fighter?

Judging from her tenure on the Suicide Squad, she's clearly a very proficient fighter.  How did this come to be?  

She was originally a psychologist, she had no training.  Was she trained by the Joker?  Is the Joker a good fighter?  I've never really been clear on this.
gpt
Harley was an excellent gymnast and in some continuities may even have gone to university on an athletic scholarship. Poison Ivy later gave her a serum that enhanced her strength, agility, and healing ability. 

Also, she's crazy. Probably helps her put 100% into the fight, with little thought to self-preservation.