U pick it up , get the achievement, and then u can drop it and quit the level.

but thier is some skulls you dont get achivments for...and some skulls ya have to hold to
end of level..

when u get the skull u dont get the achievments for all of them but just pick it up wat 10
seconds and drop it quit the level then..... when ur picking ur new lvl on the main menu
press X button to activate the ones that u wanna use

silver:
no achievment
blind,grunt birthday party,cowbell,iwhbyd

gold:
10g achievment
iron,black eye,tough luck,catch,famine,mythic,thunderstorm

u dont NEED to hold them 4 the hole lvl

you missed two skulls
fog-floodgate
tilt-cotana