No offense to the other solution but you should NOT shoot him with your ballista when the giant golem throws the huge stone and throws it at your castle so he staggers.
He should NOT stagger, the only thing that you need to do is bring up your spells and Projectile Shield just before he throws his stone, it will bounce off back to him and 1 hit kill him. Put the shield at the most left right on your door/entrance/keep just before he throws. Also no need to upgrade it, level 1 is enough.
It may take 2 or 3 tries cause I had the first time the shield didn't work and it just went tru the shield but it doesn't matter, just try again and as it bounces off he'll be dead instantly, no need to keep battling and stuff, just bring up your shield and boom :D Achievement unlocked.