I'd say 28-29 is probably a good level for beating Siofra, especially if RNG is unkind and monsters put a bunch of critical hits together. I found the pairing of Sam and Celica travelling together to be quite interesting given they borderline hate each other. Also found Maya and Metatron a little weird...but then thought about headpats!
Having gotten as far as I can storywise, I found Rosalyn both adorable and fun. Wonder if down the road she'll join up with the core group, or if she'll just stay there and wait for the MC to flirt with her because she's so adorable!
When will you be able to do more in Badon after the initial taking Metatron to the church? It still shows up but can't do anything (as of 24). Be nice to meander about and see everything, like a certain shark-girl that happens to be visiting or a modern succubus and Sarah chatting to see just how things are different (I thought about Ashley, but given Ashley's bluater that might end badly)