If i remember the description text right it says that it takes mage to control/ride the behemoth. When you summon that behemoth you also get that mage to ride it. Controlling the beast is so difficult that the mage cannot do anything else than control it. So he is not casting anything in battles.
To make it simple the spell summon one behemoth unit. There is no connection between the summoner and the unit after you get it.
I hope this answers your question
Damn im slow
Or the ninjas are just way too fast
