This was going to be too long to post but then the current wordfilter made it hilarious so I'm posting it anyway. More calculations:
Ocean of Heaven Neptune: needs 786 ATK to OHKO
----level 9 +0 SoD
The Heroes' Hideout Vampire Lord: with one Ultra Gravity, needs 1057 ATK to 2HKO
----two level 67 +0 SoD
----this is also sufficient to 2HKO Highlander
The God-King's Floating Garden Zeus: with four Ultra Gravities, needs 1171 ATK to 2HKO
----two level 92 +0 SoD
----the chimeras on the prior wave can be 2HKOed after two gravities at this point; since you're running four gravities, though, the remaining one is still pretty RNG
Super Metal Dragons Descended: Morning Star can kill a super king outright no matter what level
Tuesday Dungeon Keeper of Flame/Water/Forest (Expert): needs 752 ATK to OHKO
Tuesday Dungeon Keeper of Rainbow (Master): Assuming no gravities:
----needs 1139 ATK (level 85 SoD) to 4HKO
----needs 912 ATK (level 36 SoD) to 5HKO
----needs 761 ATK (level 41 AA) to 6HKO
----obviously if you combo hard enough you can reduce that amount, especially since you'll need to keep up healing
Wednesday Dungeon Ancient Blue Sacred Mask (Legend): with three Ultra Gravities, needs 1186 ATK to 2HKO
----two level 95 +0 SoD
----if you have either Chester you can be skillbind resistant with this setup, but I'd rather use a heartmaker and just ride it out
The Goddess Descended! Great Valkyrie (Legend): with three Ultra Gravities, needs 1174 ATK to 2HKO
----two level 92 +0 SoD
----the demons on the previous wave can be 2HKOed after one gravity at this point
The Thief Descended! Ishikawa Goemon (Legend): after three Ultra Gravities and one Wrath of God, needs 1162 ATK to 2HKO
----since you need Zeus for that strategy, you already meet that requirement
----alternatively, bring a Keeper of Rainbow to help with the bind; if you can whittle him down to half health, then with just the three gravities you need both Lucis to be at least level...32
And probably pretty important as well are skillup dungeons and biweeklies. Just a few examples because this post is getting kind of long:
Mystic Water Dragon - No Wood Antikythera:
----Legend: once it gets to Constellation Blast range, with two Ultra Gravities, needs 778 ATK to 2HKO; with one grav, needs 1107 to 2HKO
----Master: once it gets to Constellation Blast range, with one Ultra Gravity, needs 753 ATK to 2HKO; with no gravs, needs 1074
Flame Dragon Knight - No Dupes (Legend):
--------Flame Spirit Will-O'-Wisp: can be OHKOed by any Luci
--------Scorched Land Dragon Knight, Himiko: assuming the friend is a max-level Rebel Seraph Lucifer with no plusses, when she gets to Burning Yamataikoku range, one Ultra Gravity brings her to guaranteed kill range
------------with no gravity, a max level SoD + said friend will leave her at 28920 HP remaining
EVANGELION Collab The Tenth Angel - Assault Mode (Master): with three Ultra Gravities, two max level SoD leaves it at 96716 HP remaining
----you can stall as long as you need to on wave 3, and your non-gravity sub can be a darkmaker to help finish Zeruel off
----Legend might be doable with three Heras/one Zeus?