Here are some useful informations because "100% complete the game" is a bit misleading.
You need to find every collectible of course which means shiny things, all seeds and pots, all weapons and I believe all upgrades and shrines too (as they are featured next to the percentage when selecting your game save)
BUT you don't need to be fully upgraded regarding your stats as I got the achievement with three missing magic stats points (out of five).
Finishing the game with the umbrella as your only weapon "Academy of umbrellas" does not count towards this achievement (so you can get it without playing "hard mode") neither are some challenges such as "Hot Pot" and "Meal for a King". (I missed Hot Pot but still got the 100% achievement).
Following that logic, any event not linked to collectible is not needed such as "Specialist", "Big Spender" and "Conga line".
Hope this helps.
Oh and it will not pop by itself when you're done, you will have to quit to get back to the title screen, your game save will show up with a golden 100%, only then will the achievement pop!
I will update if needed.
Update: "A true ending" may be one of the conditions to get it according to @Leap of faith
Enjoy that beautiful game!