Scraps count towards acquiring the parts of the Zeke. Once you get 5 scrap pieces, you will automatically create a Zeke part.
AI pieces are made by shooting different parts of the Mech and collecting it when inside the AI pod. AI pieces make your Zeke stronger.
Also, there are custom pieces to your Zeke that you can collect. Pupa Head
Peace Walker Head
Peace Walker Leg
You can only get these pieces by fighting the Custom version of the mech. These custom pieces are even stronger than the normal Zeke parts. You only have to collect one of these pieces in order to attach it to Zeke, not 5 scrap pieces for ordinary Zeke parts. These custom pieces are the ones you need for the achievement.