I find myself with this problem from time to time as I have to remove toilets in the course of my work. Here are the ways I get around the problem: 1. Pull the toilet out completely! this lets you get at the underside with ease and this ease of access alone can help you get at the bolts with your tools. 2. Use a dremel tool or an electric drill with a thin cut-off wheel and just slice the bolt through. 3. Clamp both the nut on the underside aand the head of the bolt in the tank with Vise-Grips. If the head has no edge to grip, work it with a heavy file or cut-off wheel. 4. Drill the bolt out!!! Easy and fast! 5. If the above fails, I found that a single stick of dynamite has yet to fail. Good Luck!!