I just looked up the stats for a Carthaginian Elephant unit in export_descr_unit.txt. Here're the stats and comments from the file:

type carthaginian elephant forest
category cavalry
class heavy
stat_health 1, 10
stat_pri_armour 0, 4, 0, flesh
stat_sec_armour 10, 2, flesh

; stat_health Hit points of man, followed by hit points of mount or attached animal (if applicable)
; Ridden horses and camels do not have separate hit points
; stat_pri_armour Details of the man's defences
; armour factor
; defensive skill factor (not used when shot at)
; shield factor (only used for attacks from the front of left)
; sound type when hit = flesh, leather, or metal
;
; stat_sec_armour Details of animal's or vehicle's defenses (note riden horses do not have a separate defence)
; As per stat_pri_armour, except that the shield entry is ommited
So, the mahout has 1HP, 0 armour, 4 defense and no shield
The 'phant has 10HP and 10armour, 2 defense and no shield.

To kill an elephant you have to inflict at least 11HP at a time to get through the armour (not sure about defense?) and repeat this 10 times.
Easy, ain't it? And I was trying to kill them with a range-attack of 7 from my Parthian HA.

R'as