So!
It went pretty well while doing the dremel trick. I did a clean line in the bolt but unable to make it turn with a flat blade. After all, it has been put in place using a ratchet. I didn't used much force with the ratchet but you know that a ratchet is 1000x stronger than a flat blade.
Ho well, I guess I will have to go to a garage somewhere to get it removed. I'm not skilled enough to remove it. Not sure that I want to drill through the bolt too, as I'd like to be able to screw another bolt into that place.
Thanks for the suggestions, will try again next time it happen