Everyone plays the game differently so you can expect mixed results
Me personally, I prefered going full Mag/Atk on the damage dealers, HP/MND/DEF for the girls who wouldn't be dealing damage most of the time, aka support and tanks, and for specific characters I had different bonuses
For Reisen I had all MAG with about 20% on her speed, she was a damage dealer and a debuffer, plus she could keep herself in a better-than-average state with Grand Patriot's Elixir
For Aya i went half HP half SPD, many times she just dodged attacks and if not, she could tank one or 2, specially with an Eirin overheal
Parsee All atk, and a few times some MND, because her mind is so ridiculously high, she can make magical bosses deal 0 damage to her if you set her up correctly
If you are using Iku+Tenshi, you can have Tenshi be a decent attacker, the synergy skill she has with Iku gives a tremendous atk buff, or if you tend to keep them not together on the front row, Tenshi could be a good tank, albeit with lowish HP
Don't worry about the bosses 1shoting the non tanks, thats normal, once you get to know their attacks, and the affinities they favour, you can gear your girls better, affinity make s a huge difference in received damage
If against the orbs the problems is surviving, focus on Orb of Reverse Time as it is the most dangerous one, but then you would have to outdamage the healing and buffs of Canopy, while keeping Forward time alive, because if its the last orb on the battlefield, it will revive the other 2.
You have to pick your poison, take out their main damage dealer or the healer/buffer first, then Forward Time, then the remaining orb.
Btw Canopy is weak to silence, reducing his magical defenses to half, and Forward time to all debuffs, so you can Really make Reisen do some work in this fight, Poison also works well here.