It's pretty simple.
1. PLay Co-Op
2. Earn rewards and redeem them. You should earn quite a few fabled ores.
3. Level up the fabled ore to level 15 by combining the fabled ores together.
4. You will need a few coins but you should earn them easily by collecting the daily reward and playing the game whilst going for other achievements.
4. You get the achievement.
Any problems or suggestions with this solution post in the comments and I will rectify to reflect.