I think the general advice is to play at the level you're comfortable playing at (effort & time invested), if you're looking for farming. Friend support can be a big help. If you just want to test yourself go at it.
That being said, most of my servants are lv 60-66, and so far I'm doing okay in the qualifiers (which is the point for farming, I think). I'm planning on leveling up a bit with the lottery wins, so we'll see how far it goes haha.
Edit: In hindsight, yeah, I don't think this was the answer you were looking for. Maybe someone who is used to playing at a higher level will answer it.