Qm said

[Warhammer]

Historically elephants were done in by their morale. They also tended to not be coordinated extraordinarily well. How about reducing the maximum number of elephants in a squad? Come up with some number and the player is forced to have more commanders. With the smaller squads, more morale checks will be made, which will force them back on their own army.

Thematically, you could have the limit be based upon a size factor. With smaller troops, they are in a more compact area making them easier to control. The larger the size, the more area they take up making them more difficult to control.