Some possible sets:

Blissey (F) @ Light Clay
Ability: Healer
Tera Type: XD
EVs: 4 HP / 252 Def / 252 SpD (if no physical moves on the boss, just run max HP and SpDef)
Calm Nature
- Light Screen
- Soft-Boiled
- Rain Dance / Life Dew / Heal Bell / Helping Hand
Blissey does Blissey things, only Healer's actually going to do something for once! It has a 30% chance to cure an ally's status condition, and with Infernal Parade pretty much a given, this will be a nice passive trait. If you don't want to wait for Healer procs, Heal Bell has become much more accessible with Kitakami's reintroduction of Chimecho, who learns the move.

Gastrodon @ Covert Cloak
Ability: Sticky Hold
Tera Type: Ground
EVs: 252 HP / 4 SpA / 252 SpD
Calm Nature
- Mud-Slap
- Recover
- Clear Smog
- Earth Power
Mud-Slap for days. Clear Smog for the inevitable Calm Minds. Covert Cloak to prevent Infernal Parade from burning you. Earth Power and Tera Ground to help with shield chip and the rare SpDef drop. Wish this got Acid Spray, though :/

Manaphy @ Mystic Water / Shell Bell
Ability: Hydration
Tera Type: Water
EVs: 252 SpA / 204 SpD / 52 Spe (252/252 if not +Spe nature'd boss)
Modest Nature
IVs: 0 Atk
- Rain Dance / Take Heart
- Rest / Fake Tears
- Tail Glow
- Surf
The Rain Dance and Rest variant of this might need a bit of support to pull off, but said support (Acid Spray and SpA debuff vs boss) should be fairly common anyway. Rain Dance + Rest gives Manaphy a "Full Restore At Home", with Sleep being cured the next turn. Hydration also removes Manaphy's Burns, preventing Infernal Parade from hitting it at max power. If you want something more self-sufficient, Take Heart, Fake Tears, and Shell Bell should do the trick.