This is impossible for me. Sealion does too much damage, period. And I don't do nearly enough.
I tried with Refia, Locke, Ramza, and 2 healers.
Then again with Refia, Steiner, Sarah, Relm, and Ramza.
There's no way my healing can keep up, even with Wall, Proshellga, elemental resist gear, and 2 healers with Medica BSBs. I guess I need RES +% stuff.
Seriously, I'm capable of clearing any other content this game throws at me, but this is madness-inducing.
Sounds like you have all the tools.
I've cleared 2, and come one RNG from winning two others.
I don't always bring protectga, but I try to always have Shellga. I have 1 Major and several moderate element resist for all elements.
I use the wall that comes with it, and experimented with bringing Tyro for a third wall cast after a minute.
Resist gear doesn't seem to help much. I have a 30res ring I tried one time, didn't do crap for me.
What does help me stay alive is mBlink soul breaks like Firion and Tyro BSB's (and several medica SSBs) and then BSBs that have a heal attached, Rapha BSB Cmd 1, Ashe BSB Cmd 1. I also have Reks, and Pecil, and I think another, but haven't found use for them yet.
I really think you have right tools. Although it depends on the element really. It seems like Bismarck and Sealion are the easiest with Golem possibly being the hardest. Fenrir makes you want to throw your phone, and Hydra I have not even tried so no clue.
Edit: And all it takes is defeating one, and from there the door opens to the others since this is setup MegaMan style.