Nobody Saves the World

Note: you can skip the entirety of your NG+ playthrough by finding someone who has already completed it and join their game. You can then just defeat the final boss again in their game and you'll unlock this achievement, eliminating the need for a full second playthrough. 

Before starting your New Game+ playthrough, you should do the following on your normal playthrough:
 

• Collect all 35 mana fairies to maximize your mana (Fairy Conservationist (90G))
• Level up all Forms to S rank (Super Wiz (100G)) so they're all free to use four passives and you have all of their abilities available
• Unlock One of Everything, Please! (15G), because the shop doesn't sell Stars in NG+
• Reach max rank in all three Guilds be completing all of their quests, because enemies will all scale to your level in NG+

While you could take care of all miscellaneous achievements too (and probably should), it won't help your NG+ playthrough at all, so it's not a huge deal. 

NG+ has three major changes:
 

1. ​​​​​​All enemies always scale to your level. This isn't a huge deal since you're fully upgraded by this point. 
2. Dungeon modifiers have been rerolled. More on that shortly
3. Stars are no longer awarded for completing challenges and quests, nor can you buy them. The only way to earn stars is to complete dungeons, and you'll need to complete every dungeon in the game to complete the story

If you've done everything on your first playthrough, NG+ can focus solely on going dungeon to dungeon to complete each one, allowing you to access each Legendary Dungeon to progress the story. You'll quickly notice that the modifiers are a bit more extreme in NG+, but it's really just a matter of choosing the proper ability to counteract the modifier. Below is a list of some common/intimidating modifiers and how to easily counter them. 
 

• Damage x9999 - equip Hardened Shell so you can take six hits (fully upgraded) before dying. Also equip Zomnomnom to regain tons of health when you kill enemies
• No health drops - equip Zomnomnom so you have a way to regain health. Using Bless Me (Monk ability) is also a good way to get health if mana is plentiful
• Exploding corpses - equip Ranged Resistance to help negate damage from all of the bones flying at you from exploding corpses
• Status effects x5 and Status effects last forever - equip Poison Dampener to negate poison entirely. Otherwise, you'll have to just be careful if there are fire breathers
• Increased baddie damage of any sort - use either Melee Mitigation or Ranged Resistance as applicable

There are a lot more, but it usually pretty obvious what to use to combat the modifiers. Your passives will be geared entirely towards counteracting the modifiers and to take advantage of what's allowed. And that's the entirely of the NG+ playthrough. Just run from dungeon to dungeon. There's no point in leveling up since enemies just scale to your level anyway. You should easily have fully upgraded all of the abilities you're using by now with upgrade tokens, but continue to do so if you have more that aren't at level IV. Since your uncovered map doesn't carry over, here is a screenshot of all of the dungeons in the game, in case you aren't sure where to go next.



As far as general builds, this will vary based on playstyle, but here is what I used for almost the entirely of my NG+ playthrough:

Primary Build
 

• Necromancer (huge health amount, decent range on attack, and nonstop regular attack)
• Hat Trick to spawn Familiars nonstop, also for Sharp wards
• Gallop for Blunt wards
• Water Spray for Light wards

While there are obviously stronger abilities to do Light and Blunt attacks with, I prefer ones that have no recharge delay and use barely any mana, since that came in handy often. It's kind of pointless to list my passives since they changed for every dungeon, but my preferred ones were always:
 

• Blood Pact (can't be changed)
• Poison Tipped
• Slug's Secret
• Zomnomnom

When I didn't use the Necromancer, ​​​​​​I used the Slug. The main times I used the Slug were for dungeons where the main ward was Light damage, or if there was no melee damage allowed. My Slug build was virtually identical to my Necromancer build, except with different preferred passives.

Secondary Build
 

• Slug
• Magician's Hat to spawn Familiars and for Sharp wards
• Slime Slide for Dark wards
• Gallop for Blunt wards​​​

My passives were a bit different though:
 

• Slug's Secret (can't be changed)
• Poison Tipped
• Crit King
• Crit Cleric

With those passives I was virtually invicible with the Slug. I would crit nonstop on enemies, and it would refill so much health each time that I could literally stand still shooting and not die.