It should be a 2% chance to instant kill, but you have to remember that there are instant death multipliers on enemies. Bosses have a low multiplier, which will make it nearly impossible to kill them unless you are very lucky.

http://aigis.dougax.net/%E8%AC%8E%E3...95%B0%E5%AD%97

Good luck using her against enemies with 0.001x multipliers!