1997 Toyota Avalon catalytic converter

Hello, my name is jared I have am working on a 97 Toyota Avalon. The cat to manifold nuts and bolts are specially made to lock after installed, how do they come off without replacing the manifold? I tried heat, I tried no heat, I tried heating around the bolt in the manifold to remove the entire bolt instead of removing the nut, still no luck. They either brake or just stop moving, and when I do ever manage to get one out its all stripped. Any info. You can give me on these setups would be greatly appreciated. Thank you.