I used clove oil for a euthanasia once. I didn't have the water aerated at first, so the oil just floated on the surface, and didn't work. Once it started to mix through the water, it took several minutes to really take effect and the fish to be fully sedated. (I then added a ton more to make sure the fish didn't end up waking after I thought it was dead.) You may want to try to get him just sedated enough that he can't struggle, but not so sedated that he's totally belly up.
I hope this works for you. Do you know if the gill curl has advanced to the gill plate yet? If so, you'll have to cut it. If not, you have a chance at fixing it. It very well may take more than one session though. I've had success with it before.