Water

Jormungandr - this will be 4 water/1 dark, so I'm picking +HP over Garuda's +wind. This will also not be heavy with direct damage from abilities, so Rahab's +ability damage isn't as desired here

1 Gacha SSR kamihime used (Aphrodite), 1 story SR (Nike), 2 event SR (Gabriel/Balor)

Aphrodite - 1500 HP heal that also adds 400 HP regen over 3 turns, debuff remover that also gives self only debuff blocker, and double/triple hit buff
Nike - 750 HP heal, A-frame +12% def buff
Gabriel - A-frame -12% def debuff, B-frame +12% atk/def buff
Balor (dark!) - debuff resistance debuff, inflict poison/dizzy, inflict ...burning, I think?/blind/paralysis

Siegfried with SSR water glaive from Rahab's event as the main source of damage here, plus Sniper Shot as ex skill for B-frame -20% atk/def

Adds up to -20% atk/-32% def from debuffs (until Jormungandr comes online for another -10% atk), +24% def for mitigating damage from overdrives/raging. Plenty of healing from Aphrodite/Nike. Balor's debuffs offer some bonus damage/hopes of further damage mitigation through blind/dizzy while you're waiting for Siegfried's time to shine.