Ralvuimago has a relatively weak HP compared to the bosses you fought earlier. This battle should be easy.
The boss coils up when physically attacked
When this boss is physically attacked it coils up. If it was attacked during that time it will counter with earth attack which might be weak or strong depending on your level.
Use magic while it is coiled up
When the boss coils up attack it with magic. It won’t counter attack. This boss will only counter physical attacks while it’s coiled up.
Steal while it’s coiled up
When the boss is coiled up you have a good chance to steal from it since it won’t attack you unless you physically attack it. Ralvuimago has these items: Phoenix Down, Adaman Vest and Oak Staff.
Heal and revive while it’s coiled
When Ralvuimago coils up heal your party and revive the dead if any.
When to use Zidane’s attack
Only use Zidane’s attack when the boss is not coiled up because in such a case it would counter back with it’s powerful earth attack. However, if your level is high enough that this earth attack only deals minor damage to you then it’s OK to keep attacking it all the time. (See How to defeat Ralvurahva in Final Fantasy 9?)
Let Dagger defend
If you don’t need to heal your party then let Dagger defend. You don’t want to attack this boss with a weak attack then get a powerful counter attack. If the boss isn’t coiled up then it’s Ok to attack with Dagger.
Blizarra and Fira are effective
Both Blizarra and Fira are very effective against that boss. They can kill the boss in a few attacks. This boss is also weak against Ice. (See How to defeat Beatrix in Cleyra in Final Final 9)
How to defeat Ralvuimago in Final Fantasy 9?
How to Beat Ralvuimago in FF9? How do you beat Ralvuimago?