The Elementium Cells are a special kind of Cells.
It will be obtainable as reward, in the game events and battles, like the normal cells. But remember that Elementium is very rare! 

They can be used for the Crafting or Ranking Up of ALL the Monsters!

Crafting:
If you don’t have enough specific Cells to create a Monster you can use a part of Elementium Cell to fill the gap! But be aware that you must have at least 80 normal Cells, if you don't own the Monster, to use the rest special Cells.
Example:
Pandalf Cells owned 78-> Is NOT possible to use 22 Elementium to create the egg.
Pandalf Cells owned 85-> Is POSSIBLE to use 15 Elementium to create the egg.

Ranking Up: For the Ranking Up there are NO restrictions. You can use the amount of Elementium Cells that you desire. If you miss just 2 or 25 or ALL the normal Cells to rank up one of your Monster, you can use the Elementium Cells as substitution!