The tail swipe? If somebody has the slime debuff when it hits them then it will one shot them. The tail swipe tales like 99% HP and the Slime debuff does a DoT which will kill them. Just make sure people don't have the slime on them or stun Tail Swipe. Around that 20% it starts dumping slime on the whole party so if people get stacked with it that's when the problem starts.