It wasn't the stats that made them impossible to kill, it was the fact that the mount model had too large of a radius and infantry could not hit them. Now that they are infantry, they should be easier to kill. Think of the Trolls from TATW and that's roughly how strong they will be.
As an added bonus from the switch from elephants to infantry is the fact that trolls will no longer kill 20-30 units per swing. That will make the trolls much less deadly than before