Rarity vs Antasma: who would win?

Rarity favored — Favored (~65%)

Rarity · My Little Pony · Possessed by a Spell Book

Tier

2-A, up to LOW 1-C

Attack Potency

Multiverse Level+, up to Low Complex Multiverse Level

Speed

Subsonic

Durability

Multiverse Level+, up to Low Complex Multiverse Level

Antasma · Super Mario Bros · With the Dark Stone

Tier

2-B

Attack Potency

Multiverse Level

Speed

Massively Ftl+

Durability

Multiverse Level

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