I see Harley as able to take down Catwoman. She has guns and tons of other unpredictable gadgets. She has agility, but not as agile as Catwoman. Catwoman may be a better fighter but Harley has that unpredictability similar to that of the Joker. Plus, Harley has more lethal weapons.
Scarecrow would be immune as well. This is a rather intesting match up. Since there is no plants I will give the victory to Scarecrow since it all gets down to battle skill and he seems to be a better hand-to-hand fighter than Poison ivy. I'm not taking the Sinestro ring into consideration
I think Poison Ivy would be immune to Scarecrow's toxins. She could also create her own formula that could kill Scarecrow. Poison Ivy ftw.
When would she have the time to do this.Also Scarecrow is pretty much immune.