Cumuo-Maximus is simple and straightforward. It goes straight into an Overload Shaman deck.
Cumulo-Maximus can deal damage to an enemy hero, which is exactly what you want for your aggro gameplan.
Also, if you take into account all of the ways Shaman players can trigger dual battlecries, then this elemental will deal 10 damage. That's nothing less than a Pyroblast for five mana.