I cleared it with Double Isis
Thing about Joker is they seems to have changed his abilities and PDX hasn't taken note of it.
iirc the fight went something like
Joker appears and uses It's Showtime!
T1 Pop Ecchi, clear dummy orbs
T2 Attack
T3 Attack
T4 Joker hits me with killing joke, I pop Isis heals and attack
T5 Joker uses poison, I pop Asgard and clear poison with heart orbs
T6 Joker hits me for reduced damage, I heal/attack
T7 Joker hits me for reduced damage, I heal/attack
T8 Joker hits me for reduced damage, I heal/attack
T9 Joker poisons again, I attack without popping any poison orbs
T10 Joker was supposed to kill me here, but instead just said "4". I'm guessing it's an additional countdown to when he really uses his OHKO. However lucky for me Ecchi came back up and I used her
T11 Cleared poison with heart orbs
T12 Attack
T13 Killed him