Start by focusing the cactuars on the side. The great malboros are on the lower height level, and they can't move up to attack your team at the start. After a few turns, Nasha, Khury, and Serjes will show up on the side, where the cactuars were.
Serjes is very weak to charm, so try to put Mia close to where Serjes will spawn so you can charm him right away.
Once the trio are defeated, Shiva will appear, along with 2 white mages. She has a lot of HP, but she can be silenced, charmed, and/or slowed. Use statuses to stop Shiva for a moment and take care of the white mages to prevent their support magics.