Inferno can use a basic attack, Sobat (triple-powered Attack), Thundara,Thundaga, Gigavolt, Shockwave, Magitek Barrier, Atomic Rays, Delta Attack, and Meteor.
Ketu can use a basic attack, Slash (double-powered Attack), and Metal Cutter. Rahu can use a basic attack and Rapier, which is a basic attack with 50% more power.
Begin your battle with Hastega and Slowga (on the enemy, of course!). Using Mighty Guard may help as well. Given that Inferno has a weird idea of using attacks its weak to, put on Reflect as well.
You have no business in worrying about Rahu and Ketu; they’ll regenerate anyways. So focus on using Thundaga, Ramuh, Lightning Scrolls, etc. Have no worries about the blades’ weaknesses and such. Eventually, with a few decent Thundagas, you’ll win. Just focus them on the main body and don’t target all enemies.
After the fight, go south and through the door. Then go south and through another to head outside. Go south along the east path to find a Megalixir . Then go along the west path and bypass the door to find a chest. Open it to be able to let Party #1 go even further.
Get on the northbound conveyor belt and open the chest for a Rainbow Brush . Then circle around to the door you bypassed earlier and use it. Inside, go north to a new area. Equip some Death/Confuse-protecting equipment, then heal up. Then head over to the next dragon.