So a few days ago I thought I had what was required to get the 100% completion achievement, but i was stuck at 99% I did a search on what was needed to get to 100%, and here's a list of all the things you need to accomplish in the game

• Complete the story missions
  
• Complete all secondary missions
  
• Free all 120 serpents
  
• Complete all 24 jumps
  
• View all 32 landmarks
  
• View all 13 legends
  
• Learn all 12 solos
  
• Raise all 11 Motor Forges
  
• Raise all 24 Buried Metal Shrines
  
• Purchase all items from the Forge
  
• UNLOCK ALL CONCEPT ART (This was the one that stopped me from getting the 100%)

I'm not going to list where all the collectable items are, because there are already guides for that, but i will give you a tip. The collectables are way easier to get once you finish the story. If you finish the story, you'll receive an upgrade for your car that acts like a radar for all collectables (Serpents, jumps, landmarks, etc)

The majority of the concept art is unlocked just by completing the story.

Concept art needed to get to 100%:

• Storyboard
  
• Character
  
• Lionwhyte
  
• Landmark
  
• Ironheade : A portion of this concept art section unlocks with the story, but the rest is unlocked In the battles. You have to recruit an X amount of each unit to unlock their concept art. The weaker units require a higher number of units, than the more powerful ones.
  
  Aside from the units, there is a Megastage concept art
  
  
  It is unlocked by jumping on a level 4 stage for about 4-5 minutes during a battles (Not sure if it has to be during 1, or through several battles) Just go to the front of the stage, and jump on it like if you were performing a double team
  
  From what i read, only the Ironheade concept art is required to get the 100% (I still unlocked the concept art for the other factions, just for fun
  
  
  
• Creature (This is one of the tricky ones) In order to unlock the concept art for the creatures, you have to kill an X amount of creatures. If you are riding in the car when you kill them, it will not count towards unlocking the concept art. You have to kill them on foot. These are the creatures you need to kill a certain amount of times.
  
  Razorfire Boar
  
  
  Tollusk
  
  
  Raptor Elk
  
  
  Fan Leech
  
  
  Hextadon
  
  
  MetalQueen Chrome Recluse
  
  
  Sickle Wraith
  
  
  Guillotar
  
  
  Lazer Panther
  

Once you complete the tasks from the list, you will have 100%, If the achievement does not pop up because you had to go to multiplayer to unlock the unit concept art, go to single player and go to the stats, it will pop up the achievement