I love the behemoths, but as said Beast Masters are about impossible to code (there is not any "choice" mechanic in the game untill now).
For make something similar to your original idea, Beastmaster could have this ability:

: If you have a Behemoth in play, destroy a target permanent.
(that does pretty much the same, letting you choice what quantum to use).
Anyway, I think that, if someone makes an earth/life deck, it will just use only behemoths (couse he will probably have earth quantums to use, anyway), so maybe it's better to think something different...
Other than that, Behemoth seems quite exploitable in raimbow decks...
Maybe something like that:
Behemoth:

Train: If you have a Beastmaster in play, gain +1|+1
Beastmaster:

Behemoth pet: If you have a Behemoth in play, destroy a target element.
So they will sinergy well and they will be really good if used together.