The Morrigan vs Maiko Plasma: who would win?

Maiko Plasma favored — Slight edge (~55%)

The Morrigan · Assassin's Creed · Base

Tier
9-A, up to HIGH 8-C
Attack Potency
Small Building Level+, up to Large Building Level
Speed
Superhuman, up to Supersonic
Durability
Large Building Level

Maiko Plasma · One-Punch Man · Base

Tier
8-C
Attack Potency
Building Level
Speed
Supersonic, up to Massively Hypersonic+
Durability
Unknown

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