As you play the game, you will find treasure items which improve the damage of your traps. Some of them give a universal damage boost vs battering rams. These items are relatively rare and difficult to find, but if you keep playing you will find them eventually.

Other than that, just have plenty of traps that do lots of damage. Upgrade your damage traps so they do as much as possible. Needle and bolt traps are good for destroying everything, including battering rams.

I found an item that did 140% damage to battering rams.
That made the maps much more enjoyable, especially on maps where they're spammed a lot.