I had this team, with average of LV 75:
Power Shinki (starter), Power Kokoro, Extra Benben, Power Yuyuko, Assist Chen and Defense Keine. What I really liked about my team is that it wasn't my favorites: I wanted to look for puppets with good ranks (i.e. the letters near the stats), so it was very funny to build a balanced team. My biggest surprises were Benben and Yuyuko: Extra Benben is a beast: almost nothing can take her down. And Power Yuyuko? A troll machine with her "HP Drain" move! Long story short: if I have to restart the game, I'll be glad to use this method again.
Anyway, found reincarnation: you HAVE to beat Flandre in the basement. It's a battle versus 4 team of 3 puppets each, without rest: sounds hard, but it really isn't. Well, for me it wasn't at least. I've checked this on the Japanese wiki, and this info is nowhere to be found on the site, so I would like to share this for new players.
Maybe one day I'll do a full playthrough...