Kilia is a fire-elemental boss found in the Triet Ruins. She is weak to water and ice attacks. This is where Genis's Ice Lance will shine. Lloyd and Kratos should focus on physical attacks, while Colette can use Pow Hammer for potential stun. Kilia's attacks include fire spells and a powerful physical charge. Keep your party spread out to avoid her area-of-effect fire attacks. Raine should be on constant healing duty, especially with First Aid. If you have any items that boost water/ice damage or resist fire, equip them. Prioritize interrupting her spell casting if possible. This fight can be tough if you don't exploit her elemental weakness.
