Here is the full list of all 30 Pumpkin Jack achievements. It takes around 6-8 hours to unlock all of the achievements on Windows.
Kill the Wizard + Get the Amulet = Profit
Reach the end of the vortex like a paper boat down a drain
Get on Santa's Super-Duper-Duper-Naughty List
Solve world hunger (by killing Gluttony)
Clear away all those headstones cluttering up the graveyard
Go for a nice walk with Abelard the Gargoyle
Be the David to Dhor's Goliath (i.e. kill the big guy)
Beat the gate guards to the gate they guard on guard carts
Win the game of Conveniently-Placed Hydraulic-Operated Underground Whack-a-Mole (patent pending)
Ruin the Witch and Merchant's cruelty-free dinner
Ride the ghost horse all the way to the end
Make some magic mushroom music!
Kill the Phantom guarding the ancient mines
Pass your abandoned minecart driving test
Complete all the underground bomb-bouncing courses
Equip the Legendary Holy Glory Shovel
Find a gramophone and DANCE, DANCE, DANCE!
Finish the tutorial
Pick up a crow skull
Send a monster back to the grave
Collect all crow skulls in the fields
Smash all the crates in the fields
Kill the tyrannical Scarecrow and liberate the crows
Find all crow skulls in the ancient mines
Collect all crow skulls in the swamp
Collect all crow skulls in Skeletown
Collect all crow skulls in the graveyard
Fill out your wardrobe
Make vinyls cool again. ...Actually, that's impossible, even as a joke. Just find all the gramophones.
Give in to your inner completionist