First -
Under Completionist Skill the bottom skill Advanced Tracking allows you to select a Challenge Type to track in the Stats Menu so when youre nearby that icon you will see it on the edge of the screen. This was useful when looking for one type of challenge at a time. I suggest marking the "?" first since it has the most to collect.

Second -
Under Stats Menu there is a Submenu called Regions. This is super helpful to get 100%. I didnt even see this until after I got 100% and I facepalm myself hard.

Third -
Last Hidden Achievement is Anticlimactic. All you do is open the Achievements / Chievos Menu ingame after collecting all the challenges.

I hope these three things will help in getting 100% faster than me.