I always use the "kill" console command on Ancient Gargoyles. They're impossible, even at higher levels.
I tend to also. They are not impossible but Ancient Gargoyles just aren't worth it. And it's unrealistic. The whole Imperial Legion could not kill a small flock of them. It's just too time consuming and unrealistic, in my opinion.
I posted something on the MMM thread about having a no gargoyles pluggin like they do for some other creatures, like slims or warewolves.
http://www.gamesas.com/index.php?/topic/1116296-relz-martsmonstermod37b3-public-release-3-relz/page__view__findpost__p__16894141
To kill them I find poisons to be most effective.
I don't find gargoyles that challenging, but Balrogs. BALROGS are impossible.
I have exactly the opposite experience. Balrogs are relatively easy. Just freeze them. But Ancient Gargoyles are just unbalanced in my opinion.
Regular or lesser Gargoyles are okay. Ancient are just ridiculous.l