Mothra (Kiryu Saga) vs Arus: who would win?

Arus favored — Slight edge (~55%)

Mothra (Kiryu Saga) · Godzilla · Base

Tier
HIGH 6-A
Attack Potency
Multi-Continent Level
Speed
Supersonic+
Durability
Multi-Continent Level

Arus · Dragon Quest · End of Story

Tier
HIGH 6-A
Attack Potency
Multi-Continent Level
Speed
Massively Hypersonic+
Durability
Multi-Continent Level

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