Great Viper has two variants: Great Viper and Dragon Worm.
Now, that you have Sylphid magic, take some time to level Popoi's to level 3. You can level up Purim's Sylphid magic, but it's really not needed, unless you are going to use Balloon and/or Thunder Saber. Required level for this fight would be 21. Also, having the Midge Mallet would be nice, too.
Great Viper is very mobile, so you might have to chase him down in order to attack him physically. If you get too close to him, he can swallow you, and spit you out tiny! Use the midge mallet (or Remedy/Medical Herbs) to cure this. His other attack is a level 5 Gem Missile, which hurts - he uses this rarely, though. Assault the viper with Popoi's level 3 Thunderbolt spell (you can also use Air Blast, but it will less damage) Heal when and if you get low using Purim's level 3 Cure Water spell. This fight shouldn't last too long.
Well, this is basically Great Viper all over again, only beefier and with more attacks. Petrify Gas should not harm you as long as you have that armor from Lost Continent on. Even his level 8 Earth Slide should not harm you that much. Just like Great Viper, he's weak to wind magic, so concentrate Thunderbolts or Air Blasts on him, have Purim cast level 7 Cure Water when necessary and have Randi just attack and he will soon be a memory.