I don't have Arciela LD.... so I went Kefka. Big baddie still putting in good work
Timestamp
0:01 Phase 1 (100-79%) I did not use Trine with Kefka at the start because the Hell Wyrm will not use any HP attacks until it gets a full recast bar as long as you keep it broken. So it's a waste of his precious Trine.
The Maria manikin will do Arrow Shower on her second turn, but if she has been debuffed by light of judgement it will hit like a wet noodle.
Use OKs first free skill at the start on his AA. It needs to be up all the time to ensure Kefka can pierce through the magic resistance.
Once the hell wyrm hits a full recast bar nail him with Trine
5:08 Phase 2 (79-49%) Pretty straightforward. Make sure the boss always has hp attack disable up. Keep onion knight AA refreshed- use his LD to give him free uses of his AA.
11:45 (49-0%). At 49% boss will resummon Maria and the Lufenia orb will reappear. You can actually prevent the orb if the boss hits 49% with HP attack disable up, but in my run I let it drop. At 29% the boss will resummon Maria one last time, but you can also skip this if you bring the wyrm to 29% while it is debuffed with hp attack disable.
The maria manikin doesn't really pose a lot of threat, she will die very quickly to splash damage and Kefkas Trine also locks her.
Use the support OK and your OK LD to feed Kefka free uses of Trine. You should have enough to last to the end.
You can also check out my various strats on this stage here