Isshin vs Ferrothorn: who would win?

Ferrothorn favored — Slight edge (~55%)

Isshin · Undead Unluck · Base

Tier
HIGH 7-A
Attack Potency
Large Mountain Level
Speed
High Hypersonic, up to Massively Hypersonic+
Durability
Large Mountain Level

Ferrothorn · Pokémon · Ferrothorn

Tier
HIGH 7-A
Attack Potency
Large Mountain Level
Speed
Massively Hypersonic+
Durability
Large Mountain Level

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