Cosmic Owl vs Mephet'ran the Deceiver: who would win?

Cosmic Owl favored — Slight edge (~55%)

Cosmic Owl · Adventure Time · Base

Tier

HIGH 4-C, up to 3-C

Attack Potency

Large Star Level, up to Galaxy Level

Speed

Massively Ftl+

Durability

Large Star Level, up to Galaxy Level

Mephet'ran the Deceiver · Warhammer 40,000 · True Power/War in Heaven Deceiver

Tier

4-B, up to 1-A

Attack Potency

Solar System Level, up to Outerverse Level

Speed

Hypersonic, up to Massively Ftl+

Durability

Planet Level

A heuristic estimate from normalized VS Battles Wiki stats: tier, attack potency, speed and durability.