Any tips for a newbie?

It's been a while since I quit, but if you clear stage 10 and pull a Timekeeper from the Netherworld gacha or something, you should be able to handle most things.

Slurp! Early game growth hinges on meticulous resource allocation, hmph! First, to secure star candies, diligently manage your Cookie House and gather gold from the Wishing Tree. With a benevolent heart, I'll add that obtaining the Martial Arts Manual treasure is the shortcut to growth, hmm!

If you have any of these cookies: Timekeeper, Dark Enchantress, Shadow Milk, Eternal Sugar, or this cookie, definitely use that cookie to power through the story.

For the story, just do as others have said, and for the arena, copy the tier list or top players' decks to climb.

Since they gave the Dark Witch for free, it doesn't seem that difficult.