OK, there's a few good tips I use (isn't it annoying when the last bolt ruins). You can try to oxy the head, to expand it, or just weld a rod onto it and turn the rod to loosen the bolt. Another way is to cut a slot in the head so a screwdriver can undo it.
Yep, those bolts are a pain sometimes. I haven't done it on them but I have removed a rounded bolt by tapping a smaller size socket on with a hammer. Find a socket that's the tiniest bit smaller a knock it on, don't use a expensive socket..lol.. and you will need a new bolt when you get it out.
Weld an oversize nut on them. Spray Inox around the thread. You can get spanners that can remove rounded heads. Check Super cheap or other tools places. And in the future, use the right spanner or socket whether it's imperial or metric. They very rarely slip. Don't use adjustable wrenches as they slip.