Who could have destroyed the Dreaming Celestial?

While it was still asleep.

Parties can get creative where necessary, if not excessively cheesy. They'll need to at the very least prevent it from waking again.

Some names to start-off with:

Thanos
Doom
Sue Richards
Adam Warlock
Baron Karza
Kang
The Silver Surfer
Darkseid
John Constantine
Ares
Superman Prime (either)
The Entire Greek Pantheon (DC)
Johnny Sorrow
Odin (Marvel)
The Asgardian Destroyer
Atum
Galactus
Arishem
Scathan
Cytorrak
The Vishanti
Death (Marvel)
Mad Jim Jaspers
Merlin
The Grandmaster
The Guardians of the Universe
The Spectre
The King of Tears
The individual Endless
Lucifer
Michael

Should any of these find it too simple, how would they fare against Tiamut in his prime?

So none of these can do it then?

Nothing short of the Infinity Gauntlet ever destroyed a celestial.

imo Galactus could destroy/consume him, if he was so inclined.

Scathan and possibly MJJ.

Galactus, Scathan, Michael, Lucifer, The Spectre, Death, The full power of the Vishanti, The Endless possibly Atum