Obviously, obtaining the number of orbs, weapons, and gold medals is a given.
But be sure to physically pick up the wings, clusterduck grenades, and elemental forge to get credit for them for the achievement.
Finally, if you have everything obtained as I did and are stuck at some percentage (I had 98%), run a quick race (level 1 agility or vehicle) and it will register your progress and fill out more of the bar.
Hope this helps!