Some of the challenges are much harder than others, so I will be writing what I used to get gold. I will assume you have every transformation unlocked and Rank S (even the 2 new dlc), as well as every passive bought from the store. Some skills will need to be level 4 but many don't require. I would also highly recommend buying all the Magical Attack Increase from the store as most of the damage will be dark/light.
1. Power Defense - Pick Ghost
Passives: Zomnomnom, Blood Pact, Quick Cooldown (IV)
Skills: Water Spray, Rattler Turret, Arrow Flurry
Basically just spam turrets and keep the Boo skill going, using water spray to take down bigger enemies.
2. Break the Crystals - Pick Rat or Bee
Passives: only need Magic Life
Skills: Slime Slide (IV)
With the speed from slime slide and extra mana from magic life just hold skill and break all crystals.
3. Dark Arena - Pick Zombie or really anyone is fine
Passives: Zomnomnom, Blood Pact, Quick Cooldown (IV)
Skills: Zombite, Hat Trick
Just make some zombies and familiars and let them do the work, Stay in the Light!
4. Swift Striker - Pick Ghost
Passives: Zomnomnom, Mermaid's Resolve, Quick Cooldown (IV)
Skills: Ethereal, Arrow Flurry
Keep the Boo skill going and hold arrow flurry, pausing to use Ethereal before the boss attacks.
5. Dark Speedrun - Pick Any, I used Ghost
Passives: Don't matter
Skills: Ethereal, Slime Slide (IV)
Use Slime slide to move quickly through and Ethereal to pass through the spinning fire and daggers. During the dagger part don't go fast just hold Ethereal and go normal speed as lights turn on as you go.
6. Status Pileup - Pick Ghost
Passives: Poison Tipped, Stun Powder
Skills: Fire Breath, Zombite, Slime Slide
Just use all skills should unlock pretty easy
7. Whackamelle! - Pick Ghost
Passives: Zomnomnom, Blood Pact, Mermaid's Resolve
Skills: Zombite, Hat Trick
Keep Boo active and start zombiting enemies, than spam hat trick. Took a few tries but was able to get 132 kills.
8. Hard Shelled Baddies - Pick Ghost
Passives: Zomnomnom, Blood Pact, Mermaid's Resolve
Skills: Zombite, Fire Ball
Zombite/Boo the small enemies and Fire Ball the elites
9. Dance Dance Defender - Pick Ghost
Passives: Quick Cooldown (IV)
Skills: Water Spray, Tail Swipe, Arrow Flurry
Just be quick, you can miss 5 and still get gold so don't sweet being perfect.
10. Barrel Breaker - Pick any, I used Bee
Passives: Scavenge, Horsepower, Mermaid's Resolve
Skills: Fire Breath
Just spam Fire Breath and use signature (Jab) on barrels. Picking up any health that drops .
11. Ward Swap Boss - Pick Ghost
Passives: Mermaid's Resolve, Quick cooldown (IV)
Skills: Tails Swipe, Cloak & Dagger, Fire Ball
Use Boo at all times, break the ward than spam Fire Ball. This took a few tries and you may run out of mana if not careful.
12. Way of the Passive Fist - Pick any
Passives: Strongman, Steel Plated, Smite
Skills: Pump Up
Dummy 1 - let a bomb enemy blow it up. Dummy 2 - stand beside and have enemy hit you spreading sharp damage. Dummy 3 - Pick up health and smite with activate. Dummy 4 - Use Pump Up to hit enemy into dummy.