Because the battle with Asmodeus begins immediately after this, the party's jobs and equipment should be prepared for that before triggering the fight with King Guera. An Elementalist with Mysterio and Water-elemental abilities, along with Rock Shields and accessories to defend status ailments, should be equipped.
The party should focus on defense during this preliminary battle. King Guera frequently casts Firaga and Quara and attacks twice per turn, so an Elementalist should use Mysterio right away, and the other characters should make sure to heal each turn while also making sure they have enough AP for the Asmodeus fight. Attacking is not necessary, as the battle automatically ends at the start of the third turn.