Wait until Harley has levelled up into the triple-digits, then take her to the first DLC floor of Shukeshoo's Tower. With only her in the party, have her guard until her Fairize tension has maxed out then use her limit ability to kill all enemies and hopefully break 100,000. The calculation is that it has to be from one attack, not one hit, and so having Harley use her ability on six different enemies is how you manage to overcome this requirement.