this achievement is awarded after your first fight in chapter 7. to get here you fight as multiple heroes so the fights should be getting somewhat easy now. this can be done on any difficulty so if you want easy play on the lowest setting, "very easy"
the fights prior to this one are the following
*** Spoiler - click to reveal ***cyborg
wonder woman
superman
nightwing
poison ivy
scarecrow
joker
swamp thing
cat woman
bane
doctor fate
gorilla grodd
captain cold
deadshot
reverse flash
green lantern
aquaman
cheetah
bane (again)
atrocitus
black atom
nightwing
cyborg
supergirl
and final Harley
once you defeat harley, the achievement will pop and you can move on with the story.