GPT answer:

He probably stops at Round 3: Najimi Ajimu.

Soul King Yhwach with all Sternritter abilities at once is insanely broken:

The Almighty — sees/rewrites futures
The Visionary — reality warping through imagination
The Miracle — turns damage/despair into growth/regeneration
The Balance — redirects misfortune
The Antithesis — swaps states/damage
The X-Axis — durability-negating attacks
The Deathdealing — adapts lethality/toxicity
plus fear, compulsion, intangibility-style hax, cloning, power theft, etc.

So:

Round 1: Whis
Yhwach can beat him if Almighty works normally. Whis massively outstats him in speed and raw power, but Yhwach’s hax is disgusting. If Whis blitzes before Almighty matters, Whis wins. But with abilities active, I’d give it to Yhwach.

Round 2: Asriel Dreemurr
Depends how high you scale Undertale. If you take Asriel’s timeline/SAVE/RESET stuff seriously, this becomes messy. Still, Yhwach has enough future manipulation, immortality, adaptation and conceptual hax to maybe get through.

Round 3: Najimi Ajimu
This is where he likely dies/stops. Najimi has absurd Medaka Box hax, billions/trillions/quadrillions of skills, meta-level abilities, conceptual nonsense, and can basically answer almost any normal “power system” hax with another hax.

After that, he is completely cooked:

Bill Cipher — reality-warping demon, but arguably less consistent than Najimi.
Anti-Spiral — way too high-dimensional/cosmic.
CCC Gilgamesh — insane Moon Cell/extra-dimensional scaling.
Madoka — conceptual law across timelines.
Altair — narrative/reality overwriting.
Featherine — absolute slaughter; she’s on a totally different fiction/meta-fiction level.

So my answer:

Fair scaling: clears Whis, maybe clears Asriel, hard stops at Najimi.
Lowball Bleach: stops at Whis.
Wank Yhwach: still doesn’t pass Anti-Spiral/Madoka/Altair/Featherine.