Feolthanos is the final boss in Final Fantasy XII: Revenant Wings.
Compared to to Feolthanos Exultant, which is the second last boss and the most powerful one, Feolthanos is weak. Since all party members will be guests there is not much the player can do. Even though weak, Feolthanos's attacks are damaging and can knock out the party (depending on their levels) with one or two hits.
All the player has to do is keep attacking until Feolthanos falls. It is best to stay around Penelo as she is attacked the least and will continue to heal Vaan, even as a guest. It is likely that except for Vaan, Penelo may be the sole survivor in the battle.