Hi Duxjack - yes, the slingers can target the elephant and fire over the heads of the unit in front of them.
The modifiers to shooting would be -1 if at long range (over 9" for the slingers).
Remember that once the hits have been calculated then you must determine whether the hits are on the elephant (1,2,3,4) or the crew (5,6).
The defender then makes their saving throw.
If the elephant takes a wound then it takes a cohesion test at the end of the shooting phase using the mahout's cohesion value (if he is still alive). If the test is failed then the elephant stampedes.