I am sorry about your chickens. I lost a bunch of chickens to a bear a few months ago. We have now added an electric wire that goes around the whole coop. We already had hard tack that is even buried 12 " deep. The bear tore off the door! Anyway, you may want to add an electric fence. A few zaps and that dog may not want to come back! We have actually heard several yelps at night since we added the fence!
Speaking of that, we were lucky last night. We went to a friends for dinner before the chickens were locked up for the night. Our coop door is only 14" X 12". Right after we ate, I asked my husband and our friend to go close the coop up and turn the fence on because it was getting dark. They arrived to find a bear and her cub in the coop with all the chickens! The runner ducks were no where in sight. The bears were eating from the feeders while the chickens sat on the 6' high roosts! My husband saw the bears squeeze thru the tiny door! The mom ran up the hill, the baby ran up the nearest tree 3 feet from the coop! Ugh. So while that baby was up the tree, I went to the coop, counted the chickens and closed the door and set the electric fence. Then we hid behind a car and watched the baby slowly climb down the tree and run off to find it's mom! Then we went looking for the ducks. Finally found them down by the lake. They wouldn't come so we had to leave them there all night. They are all ok. (Runner ducks are more like chickens and go inside to sleep so this was unusual) Anyway, we are so glad that we must have gotten there right after they got inside the coop. All is ok.