Fenghuang vs Siren: who would win?

Siren favored — Slight edge (~55%)

Fenghuang · Kung Fu Panda · Base

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

Siren · God of War · Base

Tier
8-C
Attack Potency
Building Level
Speed
Sub-Relativistic+
Durability
Building Level

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