dome of protection is the default source skill for custom characters (non-origin characters). it creates a circle that regenerates physical and magic armour to characters (non-enemies) within it every time, so it's very useful for battles against bosses that have a few insta-destroy armour spells. also, are you at the same level as aetera? it's possible you're just underleveled and can come back to it after leveling once or twice; at this point of the game, one or two levels can make a big difference. make sure your gear is up to level scale as well, being unprepared may be what's causing your team to die so fast
it's a very difficult fight but it is doable
