For this achievement you need:
Find all alternate exits
Upgrade all gadgets
Complete every speedrun
Collect all 3 skulls in all levels!
You can only get 72 skulls to start with, when you have 72 skulls you can get to the hidden level, and that is where you will get the last 3 skulls in the game, it is hidden in level 2-5 'the prismatic eye. You want to choose entrance C and head up to the top where you will make your way to the last exit wich is (the hidden level)
Find all alternate exits!
You have to find all exits in all levels. Every level have atleast one exit to enter and that is when you complete the level, but some levels have more than one so simply search for exits and check your map so you can see where you have been or not been. Some exits and also skulls can only be collected from certain entrances like A B or C, to unlock entrances you have to collect skulls to unlock them.
Fully upgrade all gadgets!
You need to collect all blueprints for each level, once they are collected you can upgrade the gadgets but you will have to collect alot of loot to upgrade them, there are totally 3 upgrades for each one. You will have to spend a total of 599.000
Now over to the last thing you have to do! All levels have a speedrun you have to do, to get 100% you need to complete them for all levels. Gold, silver or bronze, you dont have to get any of the medals, just make sure you pick speedrun and finish it. There is two achievements for getting bronze and gold for all levels tho, but that does not matter for this one!
Good luck you will need it just as I did!