As non comic reader, what makes Harley Quinn so combat skilled? Did she have extensive combat experience? Don't tell me it is because she went cuckoo, she is a psychiatrist not a military woman that went nuts,
she was scissoring cops, dodging monster left and right, is this plot armor or she had some elite training because "I am just craaaazy" is not enough.
given a potion by poison ivy and train in acrobats i think is the rational for her powers.