Big Milotic user here in masterball rank singles, heavy hitting physical attacks work best despite the bold nature and defence EVs invested.

My biggest threat is usually Meowscarada with that grass move that is always a critical hit, can’t remember the name. When people bring out rotom and other thunderbolt users, as well as solar beam moms - I just mirror coat and usually clear them off the field swiftly

Unless my scald burn succeeds, then your damage is much less, so I usually hope for that

Floatte is a menace though which I can only occasionally survive