You can also insta-kill the Phantom Train in FFVI using a Pheonix Down. Sometimes it will miss but keep trying and it will work. The other useful trick is to use any item that cures the Zombie status effect. I used "revivify" to kill both the Phantom Train and Undead Behemoth in FFVI.
I didn't even think to try that in VI. I did however manage to kill Gi Nattak in VII with a Phoenix Down though. It did take five attempts to do so, therefore if you are overpowered for the area, I'd recommend just laying waste to him old school style. If you are underleveled like I was, "Hold onto your butts."