Something along the lines of Inspiration and Fear. After all you are either fighting alongside a giant war elephant or against a huge armoured monster.
So possibly a non stacking Fear aura that reduces the hp of all enemy living units(non siege) by 5% within a radius.
And increases the hp of all living units by 5% within a radius.
Either way the elephant is not performing adequate for it’s cost, and is generally a distraction carnifax for the cost. And instead of increasing its strength in the cases where they might already be performing well(like against someone who didnt realise they should make xbows/HC/springs), rather give it more utility.