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When you click on the Achievement Cup, it tells you how many hands to collect at each level.
A TOUCH OF HALLOWEEN ACHIEVEMENT
LEVEL.....52 HANDS REQ.....
Level 1............ 3
LEVEL 2........... 4
LEVEL 3........... 5
LEVEL 4........... 6
LEVEL 5........... 7
LEVEL 6........... 8
LEVEL 7........... 9
LEVEL 8 ........... 10
There's 41 hands in the Main Story, so that means you'll have to get 11 more Hands from the Challenges in order to get the Achievement.
Challenge 1 has 10 hands
Challenge 2 has 11 hands
Challenge 3 has 12 hands
You can just clear paths to the hands in the Challenges if you choose but you might miss some good materials if you do that. Plus you should consider getting all 74 hands to buy the Event equipment, as its usually better than the regular Shop.
http://wiki.diggysadventure.com/index.php?title=Events#Halloween_2018_.28Segmented_Event.29
