Saga Frontier Remastered Optional Boss: King Sei
3I'm putting this fight down as optional because unless you're on NG+ don't fight this guy on your first visit.
He'll always open with his entire squadron attacking you for probably around 500 damage. I believe this is to stop you from beating him in ACT 1 of Red's story when you're just supposed to travel around the Cygnus.
Not only does this boss do some serious damage but his Skeleton Knights can be a pest. He also has Deathsynthesis meaning he'll be regenerating health every turn so you need to find time to hurt him while keeping yourself topped off or you'll never win.
Regardless of being on New Game or New Game + I highly recommed Mei Ling for this fight thanks to her guns doing alright damage and her knowing sunray when she joins which does increased damage against king Sei.