Metal Mystic vs Carlotta Montelli: who would win?

Carlotta Montelli favored — Slight edge (~55%)

Metal Mystic · Teenage Mutant Ninja Turtles · True Form

Tier
5-B, up to 5-A
Attack Potency
Planet Level+, up to Large Planet Level
Speed
Hypersonic
Durability
Planet Level+, up to Large Planet Level

Carlotta Montelli · Wuthering Waves · Base

Tier
5-A
Attack Potency
Large Planet Level
Speed
Sub-Relativistic+
Durability
City Level

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