Sylveon's main set is Choice Specs, and the main coverage move before the first slash is Hidden Power Fire, which is a 2hko against SpDef Skarmory.
The analysis set runs only 16spe, so Skarmory could be at least an NSI Check to that, 2hkoing with Iron head.
I'd even go as far as to say that Choice Specs is Sylveon's only non-outclassed set.
This is, however, poorly reflected in Sylveon's usage stats: only 42.753% of Sylveons run Choice Specs in 1825 usage. Assuming that only Specs Sylveon runs HP Fire, this still means that 50.315% of Specs Sylveons have something for SDef Skarmory.
Verdict: Either SSI or NSI based on the analysis, SSI based on the usage stats?
The analysis set runs only 16spe, so Skarmory could be at least an NSI Check to that, 2hkoing with Iron head.
I'd even go as far as to say that Choice Specs is Sylveon's only non-outclassed set.
This is, however, poorly reflected in Sylveon's usage stats: only 42.753% of Sylveons run Choice Specs in 1825 usage. Assuming that only Specs Sylveon runs HP Fire, this still means that 50.315% of Specs Sylveons have something for SDef Skarmory.
Verdict: Either SSI or NSI based on the analysis, SSI based on the usage stats?