With the last digit of Robo's HP as a 9, he can deal over 6000 damage with Rapid-fire Fist.
Even with the Dragon Earring/Tear, the Apocalypse Arm has a mere 30% critical hit rate... That's about one-third.
So, (approximately) you could have Robo dealing ~10000 damage every three turns. (on average) With the Crisis Arm/Rapid-fire Fist combo, however, Robo can deal 18000+ damage every three turns.
Crisis Arm/Rapid-fire Fist > Apocalypse Arm/Dragon Earring.