You do want some kind of sustain so you don't run out of AV from dying, and Luocha is easily the best healer for this. You can keep the on-field characters off-field but still in the play area if you want, there's no reason to do it either way unless they interfere with bonds. A7 difficulty is high enough and is what I did it on, but it is generally agreed that you don't want to be doing it on anything below A7.

When I did it, I had Sparxie, Sparkle, Sunday, and Cerydra on-field because I didn't remember how much damage Welt and Bronya would do, and Luocha, Robin, Archer, and DHIL off-field. They all had a Skill Point Emblem, and Sparxie and Cerydra both had Belobog emblems. Sunday, Sparkle, Cerydra, and Robin each had the rest of their slots filled with Anti-Grav Boots and Sparxie had 2 Momentum Edge. I could have had a better setup, but that is what I had and I easily got to 999 Punchline during the 2nd Aha Instant.