Roy Alphard vs Abby: who would win?

Abby favored — Slight edge (~55%)

Roy Alphard · Re:Zero kara Hajimeru Isekai Seikatsu · Base

Tier

7-B, up to 7-A

Attack Potency

City Level, up to Mountain Level

Speed

Superhuman, up to Relativistic+

Durability

City Level+, up to Mountain Level

Abby · Wuthering Waves · Base

Tier

7-B, up to 5-A

Attack Potency

City Level, up to Large Planet Level

Speed

Sub-Relativistic+

Durability

City Level, up to Large Planet Level

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