Buffer Erika

The most powerful and efficient buff is sieglinde+rally Def and Res, together with her native hone speed.
This frees up her sacred seal slot, for any trick you might want to give her, which really improves her performance.

Pretty much the above.

However, the downside to using a split-rally skill is that the units you don't rally will only receive +7 to their stats.

If you're running more than one blade user. Use Rally Def with the Fortify Res seal.

I'd also suggest you inherit Rally Res so you can switch between them since Fortify Def seal is now a thing.