Whoa. The dragons at Mt. Nibel have a gravity based physical attack. gjoe, was this on purpose or was it an oversight? This makes them extremely easy to kill.
Start out with sneak attack + mystify to get him berserked. Have a female character equipped with minerva bangle with about 4k or so current HP and 30k max HP, with mastered cover materia. As soon as possible, cast dragon force X2 (or 4 if you can) on the female character to make sure she can survive his other physical attack which apparently isn't gravity based. Pair a mime with command counter on female party member and have her cast

?. The minerva bangle ensures his gravity attack won't hurt the female character (I chose minerva bangle since you can't ABSORB the gravity, or it will render

? useless), and his other physical attack actually helps since it increases the effectiveness of

?. The 4k HP is important, since you will suffer about 1.5k to 2k damage or so from his physical attacks total by the end of the battle. If you have the game cursor set to memory, all you have to do is hold down X and he's dead.