Even better idea...

MOD THEM! If you're into historical accuracy then this really isn't cheating... well yes it is but if you're really sick of these ancient pachyderms behaving like modern main battle tanks then what other option is there?

Forest War and Armored elephants possess an obscene number of Hit Points, 12, 15 and 15 to be exact. So even if your troops punch through their Armor (10, 13 & 16 respectively) you still have to knock those HPs down to kill a single beast. It's ridiculous. I cannot imagine what it must be like to face down 5 units of unmodded Armored Elephants, especially if they're experienced and possess weapons and armor upgrades! You should not have to waste such a huge portion of your force to handle these beasts!

I modded the Hit Points of all elephants down to 3 each. I also lowered their Armor ratings and purchase costs. My modded elephants are still tough and can still ruin your party but the days of these beasts going where they please with virtual impunity are over! Given sufficient time, the missiles of most skirmishers and archer/horse archer units will now have a noticeable effect on elephants instead of being utterly useless against them with their default stats.

As an experiment I am also tempted to try raising their Hit Points back to ridiculous levels and lowering their Armor ratings even further but I feel that will make them too easy to kill at a distance with prolonged missile volleys.