Space Ishtar vs Psaro the Manslayer: who would win?

Space Ishtar favored — Slight edge (~55%)

Space Ishtar · Nasuverse · Prime Ishtar

Tier
LOW 2-C, up to 1-C
Attack Potency
Universe Level+, up to Complex Multiverse Level
Speed
Omnipresent
Durability
Universe Level+, up to Complex Multiverse Level

Psaro the Manslayer · Dragon Quest · Base

Tier
LOW 2-C
Attack Potency
Universe Level+
Speed
Immeasurable
Durability
Universe Level+

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