Yeah, completionism is definitely a thing. I haven't tried Hard Mode yet...I want to go in there with a team of all 60s and I'm still some way away from that with a couple of my maidens. Challenge mode does frustrate me a bit though, since I'm having to level maidens I'd never use in Hard Mode because of the team composition restrictions. I may just end up ignoring all the 'Only Nature Maidens' challenges because Nuka sucks ass compared to...well, almost anyone else, and I don't have either of the other two nature maidens.
You need more than a team of 60s for hard.
Haven't tried it for a while, but in normal mode I have all 60 stages at 3* complete, but only 3 stages of hard mode at 3* and the 4th stage at 2*. I could maybe make it to stage 10 now with equipment and building upgrades I have done since.
As for challenge mode, I have 57/60 complete.
What is a good stage to farm to level up girls?
Boss fights give the best experience and coins per energy. Player levels are best obtained from HM1 or HM21 onwards (non-boss), since they give 500 exp per 10 energy.
Hard mode requires smart play. Keep your girls spread out so AoE attacks don't wipe them. Pick them up before they get hit by a ranged attack to keep them safe. Send ones low on health to the back if you can't heal them immediately. Level up their skills and use them appropriately, including using manual targeting when available. I've cleared stage 58 this way, and my team is Helen, Althea, Nuka, and Luna. (Stage 59 has two necromancers at the end that keep spamming skeletons which my team keeps prioritizing instead of the totem, causing time to run out.)
The nice thing about hard mode is the equipment drop rate on the later stages. Stage 53 is best with a 70% and 68% (IIRC) for two rares.
Any ideas when pvp (arena?) coming and what it would look like?
What I've observed is that when I see an arrow heading towards a girl, if I pick her up, wait for the arrow to hit and disappear, and then set the girl back down, then there will be no damage number over her head and I won't see her health bar visibly decrease. But if I put her down somewhere else before the arrow disappears, then it will curve mid-flight, hit her and cause damage. This appears to be true for both normal and special attacks. I just tested this fighting some archers, so maybe it behaves differently with magic missiles.