I'd have to come up with some way of building walls with pockets of water and magma sealed within them, so that in the event of a wall breach the destroyed wall segment would be instantly recreated in Obsidian.
And because, you know, Magma.
2 walls with one tile thick hollow in between. Pump magma between walls, build floor and carve fortifications on top. Wallbreaker gets magma bath when it does its thing. Water optional.
I don't think a cooldown suits very well to DF. But the wall destruction should be slow on stronger materials. A wooden fence should not be very effective against huge brutes but a massive stone or steel wall should give you enough time to prepare traps, troops and magma.
If destroying a wall would take about as long as it does for a dabbling mason to deconstruct it, I suppose a cooldown would just make it frustrating for the destroyer.
"Okay, finally busted down this wall, time for the one right behind it."
*Cooldown in progress, 20 minutes remaining*
" ...What do you mean 20 minutes!?"
*20 minutes later, wall is destroyed, another wall behind that one*
"OH COME ON."
I just have a habit of making my fort almost entirely out of constructed walls, and don't want one of these things acting like a bull in a china shop when there are still perfectly healthy dwarves to murder. That's all.