So. I'm on my fourth newgame+ now. Here are a few things i've noticed.
With every newgame+ you do, the chance of higher quality rare bonus increases. I've been playing on nothing but normal difficulty,haven't done hard mode yet, but everytime i do a skill upgrade now,almost every single one of them is a red+ on a skill upgrade. I'm also starting to see two yellow ++ for blue passives, which is nice when you start getting 2 bonus affixes to stuff like physical resist, reducing the deck cost and reducing damage from another type.
The goddess's skills are absurd powerful. Judgement arrow is a light element "Acid rain" with ZERO zone control, but hits like a truck. It can one shot labyrinth (goth girl) before her first down on 1 and 2 heart gems. Even against Arnesta (red hair) and Aquila (purple hair), which the skill is weak against,i've seen it take out 50% of their hp. I find that i end up shelving judgement arrow and never using it because of how absurdly powerful it is. It does WAY too much damage for just "70" damage,it feels more like 70 x 3 hits or something.
Sacred oracle...oh boy. Where do i start with this skill. If you've fought labyrinth on her second or third heart gem (where she's at full power), you'll know just how much gauge control she does with lunatic waltz and gauge removal. She can pretty much prevent you from doing any actions if you don't have any quick skills or obtained defensive skills against lunatic waltz/gauge removal.
Sacred oracle is gauge removal on steroids. Paired up with your own lunatic waltz,you can essentially run up against any 1-2 heart gem heroines and stall them for eternity.(This is ESPECIALLY noticeable when you chug down a speed potion and don't down any of them once.) You can then pair up Healing skill obtained from Aquila(Purple hair), and smile flash from Citron(the blond haired magical girl) and you're essentially immortal while tickling them to death. Despite having 0 damage on Smile flash, it still does damage.(literally death by a thousand cuts though). This is GREAT for leveling up skills to max cooldown reduction in a single fight, but not great for your sanity and your PC, as smile flash can lag slower computers with how....graphically intense it is.
do NOT put healing and full boost skills together. At all. Full boost is bugged and for some reason,and resets the cooldown of healing whenever cast, effectively making healing a wasted slot in that battle.
You can have a max of 5 slots per deck, with a max deck cost of 24. Note that it costs 120 expansion points to go from 20 --> 24 deck cost, and going from the 4th ---> 5th deck slot costs 250 Expansion points.
Lastly, acid rain is the most effective skill in the game. You could probably even cheese it and run a deck of nothing but 3-4 acid rains and clear the game just that way. Acid rain offers 7% zone control everytime it's cast, and is the fastest skill in the game, especially if you master it and lower it's cooldown to 6 seconds. It does low damage,but with how spammable it is, it could even outdps some of the higher damage skills.
Zone control is important in this game. It's not very apparent when you first start out, but higher zone control reduces the amount of damage you take. When you sacrifice zone control to deal corruption damage to heart gems, note that you'll start taking increased damage when the girls escape the bind. If you ever wonder why you start dying so much, try not sacrificing all the zone control in a single bind. Sometimes it might be better to do small amounts of corruption on the first or second down, and then get as much zone control as you can, then blow it all on their third down. (Or second down,if you're still on the 1st or 2nd heart gem, since they pretty much are guaranteed to run when corruption is above 30% and they've been downed twice in early stages.)